[MIPS] Use generic isBlockOnlyReachableByFallthrough (llvm#80799)

FastISel may create a redundant BGTZ terminal which fallthroughes.
```
  BGTZ %2:gpr32, %bb.1, implicit-def $at

bb.1.bb1:
; predecessors: %bb.0
```

The `!I->isBarrier()` check in
MipsAsmPrinter::isBlockOnlyReachableByFallthrough
will incorrectly not print a label, leading to a `Undefined temporary
symbol `
error when we try assembling the output assembly file. See the updated
`Fast-ISel/pr40325.ll` and
rust-lang/rust#108835

In addition, the `SwitchInst` condition is too conservative and prints
many unneeded labels (see the updated tests).

Just use the generic isBlockOnlyReachableByFallthrough, updated by
commit 1995b9f for SPARC, which also
handles MIPS.

(cherry picked from commit 6b2fd7a)

llvm/lib/Target/Mips/MipsAsmPrinter.cpp

@@ -471,45 +471,6 @@ void MipsAsmPrinter::emitBasicBlockEnd(const MachineBasicBlock &MBB) {
     TS.emitDirectiveInsn();
 }
 
-/// isBlockOnlyReachableByFallthough - Return true if the basic block has
-/// exactly one predecessor and the control transfer mechanism between
-/// the predecessor and this block is a fall-through.
-bool MipsAsmPrinter::isBlockOnlyReachableByFallthrough(const MachineBasicBlock*
-                                                       MBB) const {
-  // The predecessor has to be immediately before this block.
-  const MachineBasicBlock *Pred = *MBB->pred_begin();
-
-  // If the predecessor is a switch statement, assume a jump table
-  // implementation, so it is not a fall through.
-  if (const BasicBlock *bb = Pred->getBasicBlock())
-    if (isa<SwitchInst>(bb->getTerminator()))
-      return false;
-
-  // If this is a landing pad, it isn't a fall through.  If it has no preds,
-  // then nothing falls through to it.
-  if (MBB->isEHPad() || MBB->pred_empty())
-    return false;
-
-  // If there isn't exactly one predecessor, it can't be a fall through.
-  if (MBB->pred_size() != 1)
-    return false;
-
-  // The predecessor has to be immediately before this block.
-  if (!Pred->isLayoutSuccessor(MBB))
-    return false;
-
-  // If the block is completely empty, then it definitely does fall through.
-  if (Pred->empty())
-    return true;
-
-  // Otherwise, check the last instruction.
-  // Check if the last terminator is an unconditional branch.
-  MachineBasicBlock::const_iterator I = Pred->end();
-  while (I != Pred->begin() && !(--I)->isTerminator()) ;
-
-  return !I->isBarrier();
-}
-
 // Print out an operand for an inline asm expression.
 bool MipsAsmPrinter::PrintAsmOperand(const MachineInstr *MI, unsigned OpNum,
                                      const char *ExtraCode, raw_ostream &O) {

llvm/lib/Target/Mips/MipsAsmPrinter.h

@@ -142,8 +142,6 @@ class LLVM_LIBRARY_VISIBILITY MipsAsmPrinter : public AsmPrinter {
   void emitFunctionBodyStart() override;
   void emitFunctionBodyEnd() override;
   void emitBasicBlockEnd(const MachineBasicBlock &MBB) override;
-  bool isBlockOnlyReachableByFallthrough(
-                                   const MachineBasicBlock* MBB) const override;
   bool PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,
                        const char *ExtraCode, raw_ostream &O) override;
   bool PrintAsmMemoryOperand(const MachineInstr *MI, unsigned OpNum,

llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll

@@ -11,7 +11,7 @@ define void @test(i32 %x, ptr %p) nounwind {
 ; CHECK-NEXT:    andi $1, $4, 1
 ; CHECK-NEXT:    bgtz $1, $BB0_1
 ; CHECK-NEXT:    nop
-; CHECK-NEXT:  # %bb.1: # %foo
+; CHECK-NEXT:  $BB0_1: # %foo
 ; CHECK-NEXT:    jr $ra
 ; CHECK-NEXT:    nop
   %y = and i32 %x, 1

llvm/test/CodeGen/Mips/GlobalISel/llvm-ir/jump_table_and_brjt.ll

@@ -25,7 +25,7 @@ define i32 @mod4_0_to_11(i32 %a) {
 ; MIPS32-NEXT:    sltu $1, $1, $2
 ; MIPS32-NEXT:    bnez $1, $BB0_6
 ; MIPS32-NEXT:    nop
-; MIPS32-NEXT:  $BB0_1: # %entry
+; MIPS32-NEXT:  # %bb.1: # %entry
 ; MIPS32-NEXT:    lw $2, 28($sp) # 4-byte Folded Reload
 ; MIPS32-NEXT:    lui $1, %hi($JTI0_0)
 ; MIPS32-NEXT:    sll $2, $2, 2
@@ -65,7 +65,7 @@ define i32 @mod4_0_to_11(i32 %a) {
 ; MIPS32-NEXT:    sltu $1, $1, $2
 ; MIPS32-NEXT:    bnez $1, $BB0_13
 ; MIPS32-NEXT:    nop
-; MIPS32-NEXT:  $BB0_8: # %sw.epilog
+; MIPS32-NEXT:  # %bb.8: # %sw.epilog
 ; MIPS32-NEXT:    lw $2, 0($sp) # 4-byte Folded Reload
 ; MIPS32-NEXT:    lui $1, %hi($JTI0_1)
 ; MIPS32-NEXT:    sll $2, $2, 2
@@ -125,7 +125,7 @@ define i32 @mod4_0_to_11(i32 %a) {
 ; MIPS32_PIC-NEXT:    sltu $1, $1, $2
 ; MIPS32_PIC-NEXT:    bnez $1, $BB0_6
 ; MIPS32_PIC-NEXT:    nop
-; MIPS32_PIC-NEXT:  $BB0_1: # %entry
+; MIPS32_PIC-NEXT:  # %bb.1: # %entry
 ; MIPS32_PIC-NEXT:    lw $2, 8($sp) # 4-byte Folded Reload
 ; MIPS32_PIC-NEXT:    lw $3, 36($sp) # 4-byte Folded Reload
 ; MIPS32_PIC-NEXT:    lw $1, %got($JTI0_0)($2)
@@ -167,7 +167,7 @@ define i32 @mod4_0_to_11(i32 %a) {
 ; MIPS32_PIC-NEXT:    sltu $1, $1, $2
 ; MIPS32_PIC-NEXT:    bnez $1, $BB0_13
 ; MIPS32_PIC-NEXT:    nop
-; MIPS32_PIC-NEXT:  $BB0_8: # %sw.epilog
+; MIPS32_PIC-NEXT:  # %bb.8: # %sw.epilog
 ; MIPS32_PIC-NEXT:    lw $2, 8($sp) # 4-byte Folded Reload
 ; MIPS32_PIC-NEXT:    lw $3, 4($sp) # 4-byte Folded Reload
 ; MIPS32_PIC-NEXT:    lw $1, %got($JTI0_1)($2)

llvm/test/CodeGen/Mips/compactbranches/unsafe-in-forbidden-slot.ll

@@ -18,15 +18,15 @@ sw.bb:                                            ; preds = %entry
   br label %sw.epilog
 ; CHECK: beqzc
 ; CHECK-NEXT: nop
-; CHECK-NEXT: .LBB
+; CHECK-NEXT: # %bb.1
 ; CHECK-NEXT: j
 
 sw.bb1:                                           ; preds = %entry, %entry
   store volatile i32 2, ptr @boo, align 4
   br label %sw.epilog
 ; CHECK: bnezc
 ; CHECK-NEXT: nop
-; CHECK-NEXT: .LBB
+; CHECK-NEXT: # %bb.3
 ; CHECK-NEXT: j
 
 sw.epilog:                                        ; preds = %entry, %sw.bb1, %sw.bb

llvm/test/CodeGen/Mips/indirect-jump-hazard/jumptables.ll

@@ -42,7 +42,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; MIPS32R2-NEXT:    sltiu $1, $4, 7
 ; MIPS32R2-NEXT:    beqz $1, $BB0_6
 ; MIPS32R2-NEXT:    sw $4, 4($sp)
-; MIPS32R2-NEXT:  $BB0_1: # %entry
+; MIPS32R2-NEXT:  # %bb.1: # %entry
 ; MIPS32R2-NEXT:    sll $1, $4, 2
 ; MIPS32R2-NEXT:    lui $2, %hi($JTI0_0)
 ; MIPS32R2-NEXT:    addu $1, $1, $2
@@ -100,7 +100,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; MIPS32R6-NEXT:    sltiu $1, $4, 7
 ; MIPS32R6-NEXT:    beqz $1, $BB0_6
 ; MIPS32R6-NEXT:    sw $4, 4($sp)
-; MIPS32R6-NEXT:  $BB0_1: # %entry
+; MIPS32R6-NEXT:  # %bb.1: # %entry
 ; MIPS32R6-NEXT:    sll $1, $4, 2
 ; MIPS32R6-NEXT:    lui $2, %hi($JTI0_0)
 ; MIPS32R6-NEXT:    addu $1, $1, $2
@@ -159,7 +159,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; MIPS64R2-NEXT:    sltiu $1, $2, 7
 ; MIPS64R2-NEXT:    beqz $1, .LBB0_6
 ; MIPS64R2-NEXT:    sw $4, 4($sp)
-; MIPS64R2-NEXT:  .LBB0_1: # %entry
+; MIPS64R2-NEXT:  # %bb.1: # %entry
 ; MIPS64R2-NEXT:    dsll $1, $2, 3
 ; MIPS64R2-NEXT:    lui $2, %highest(.LJTI0_0)
 ; MIPS64R2-NEXT:    daddiu $2, $2, %higher(.LJTI0_0)
@@ -254,7 +254,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; MIPS64R6-NEXT:    sltiu $1, $2, 7
 ; MIPS64R6-NEXT:    beqz $1, .LBB0_6
 ; MIPS64R6-NEXT:    sw $4, 4($sp)
-; MIPS64R6-NEXT:  .LBB0_1: # %entry
+; MIPS64R6-NEXT:  # %bb.1: # %entry
 ; MIPS64R6-NEXT:    dsll $1, $2, 3
 ; MIPS64R6-NEXT:    lui $2, %highest(.LJTI0_0)
 ; MIPS64R6-NEXT:    daddiu $2, $2, %higher(.LJTI0_0)
@@ -351,7 +351,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; PIC-MIPS32R2-NEXT:    sltiu $1, $4, 7
 ; PIC-MIPS32R2-NEXT:    beqz $1, $BB0_6
 ; PIC-MIPS32R2-NEXT:    sw $4, 4($sp)
-; PIC-MIPS32R2-NEXT:  $BB0_1: # %entry
+; PIC-MIPS32R2-NEXT:  # %bb.1: # %entry
 ; PIC-MIPS32R2-NEXT:    sll $1, $4, 2
 ; PIC-MIPS32R2-NEXT:    lw $3, %got($JTI0_0)($2)
 ; PIC-MIPS32R2-NEXT:    addu $1, $1, $3
@@ -413,7 +413,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; PIC-MIPS32R6-NEXT:    sltiu $1, $4, 7
 ; PIC-MIPS32R6-NEXT:    beqz $1, $BB0_6
 ; PIC-MIPS32R6-NEXT:    sw $4, 4($sp)
-; PIC-MIPS32R6-NEXT:  $BB0_1: # %entry
+; PIC-MIPS32R6-NEXT:  # %bb.1: # %entry
 ; PIC-MIPS32R6-NEXT:    sll $1, $4, 2
 ; PIC-MIPS32R6-NEXT:    lw $3, %got($JTI0_0)($2)
 ; PIC-MIPS32R6-NEXT:    addu $1, $1, $3
@@ -476,7 +476,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; PIC-MIPS64R2-NEXT:    sltiu $1, $3, 7
 ; PIC-MIPS64R2-NEXT:    beqz $1, .LBB0_6
 ; PIC-MIPS64R2-NEXT:    sw $4, 4($sp)
-; PIC-MIPS64R2-NEXT:  .LBB0_1: # %entry
+; PIC-MIPS64R2-NEXT:  # %bb.1: # %entry
 ; PIC-MIPS64R2-NEXT:    dsll $1, $3, 3
 ; PIC-MIPS64R2-NEXT:    ld $3, %got_page(.LJTI0_0)($2)
 ; PIC-MIPS64R2-NEXT:    daddu $1, $1, $3
@@ -539,7 +539,7 @@ define ptr @_Z3fooi(i32 signext %Letter) {
 ; PIC-MIPS64R6-NEXT:    sltiu $1, $3, 7
 ; PIC-MIPS64R6-NEXT:    beqz $1, .LBB0_6
 ; PIC-MIPS64R6-NEXT:    sw $4, 4($sp)
-; PIC-MIPS64R6-NEXT:  .LBB0_1: # %entry
+; PIC-MIPS64R6-NEXT:  # %bb.1: # %entry
 ; PIC-MIPS64R6-NEXT:    dsll $1, $3, 3
 ; PIC-MIPS64R6-NEXT:    ld $3, %got_page(.LJTI0_0)($2)
 ; PIC-MIPS64R6-NEXT:    daddu $1, $1, $3

llvm/test/CodeGen/Mips/jump-table-mul.ll

@@ -10,7 +10,7 @@ define i64 @test(i64 %arg) {
 ; CHECK-NEXT:    sltiu $1, $4, 11
 ; CHECK-NEXT:    beqz $1, .LBB0_4
 ; CHECK-NEXT:    nop
-; CHECK-NEXT:  .LBB0_1: # %entry
+; CHECK-NEXT:  # %bb.1: # %entry
 ; CHECK-NEXT:    daddiu $1, $2, %lo(%neg(%gp_rel(test)))
 ; CHECK-NEXT:    dsll $2, $4, 3
 ; CHECK-NEXT:    ld $3, %got_page(.LJTI0_0)($1)

llvm/test/CodeGen/Mips/pseudo-jump-fill.ll

@@ -14,7 +14,7 @@ define i32 @test(i32 signext %x, i32 signext %c) {
 ; CHECK-NEXT:    sltiu $1, $5, 4
 ; CHECK-NEXT:    beqz $1, $BB0_6
 ; CHECK-NEXT:    addu $3, $2, $25
-; CHECK-NEXT:  $BB0_1: # %entry
+; CHECK-NEXT:  # %bb.1: # %entry
 ; CHECK-NEXT:    li16 $2, 0
 ; CHECK-NEXT:    sll16 $5, $5, 2
 ; CHECK-NEXT:    lw $6, %got($JTI0_0)($3)