[CGP] Drop poison-generating flags after hoisting (#90382)

See the following case:
```
define i8 @SRC1(i8 %x) {
entry:
  %cmp = icmp eq i8 %x, -1
  br i1 %cmp, label %exit, label %if.then

if.then:
  %inc = add nuw nsw i8 %x, 1
  br label %exit

exit:
  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
  ret i8 %retval
}

define i8 @tgt1(i8 %x) {
entry:
  %inc = add nuw nsw i8 %x, 1
  %0 = icmp eq i8 %inc, 0
  br i1 %0, label %exit, label %if.then

if.then:                                          ; preds = %entry
  br label %exit

exit:                                             ; preds = %if.then, %entry
  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
  ret i8 %retval
}
```
`optimizeBranch` converts `icmp eq X, -1` into cmp to zero on RISC-V and
hoists the add into the entry block. Poison-generating flags should be
dropped as they don't still hold.

Proof: https://alive2.llvm.org/ce/z/sP7mvK
Fixes #90380

(cherry picked from commit ab12bba)

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -8154,6 +8154,7 @@ static bool optimizeBranch(BranchInst *Branch, const TargetLowering &TLI,
       IRBuilder<> Builder(Branch);
       if (UI->getParent() != Branch->getParent())
         UI->moveBefore(Branch);
+      UI->dropPoisonGeneratingFlags();
       Value *NewCmp = Builder.CreateCmp(ICmpInst::ICMP_EQ, UI,
                                         ConstantInt::get(UI->getType(), 0));
       LLVM_DEBUG(dbgs() << "Converting " << *Cmp << "\n");
@@ -8167,6 +8168,7 @@ static bool optimizeBranch(BranchInst *Branch, const TargetLowering &TLI,
       IRBuilder<> Builder(Branch);
       if (UI->getParent() != Branch->getParent())
         UI->moveBefore(Branch);
+      UI->dropPoisonGeneratingFlags();
       Value *NewCmp = Builder.CreateCmp(Cmp->getPredicate(), UI,
                                         ConstantInt::get(UI->getType(), 0));
       LLVM_DEBUG(dbgs() << "Converting " << *Cmp << "\n");

llvm/test/Transforms/CodeGenPrepare/ARM/branch-on-zero.ll

@@ -211,6 +211,29 @@ else:
   ret i32 %l
 }
 
+define i32 @sub10_else_drop_nuw(i32 %a) {
+; CHECK-LABEL: @sub10_else_drop_nuw(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[L:%.*]] = sub i32 [[A:%.*]], 10
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i32 [[L]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[THEN:%.*]], label [[ELSE:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    ret i32 0
+; CHECK:       else:
+; CHECK-NEXT:    ret i32 [[L]]
+;
+entry:
+  %c = icmp eq i32 %a, 10
+  br i1 %c, label %then, label %else
+
+then:
+  ret i32 0
+
+else:
+  %l = sub nuw i32 %a, 10
+  ret i32 %l
+}
+
 define i32 @subm10_then(i32 %a) {
 ; CHECK-LABEL: @subm10_then(
 ; CHECK-NEXT:  entry:

llvm/test/Transforms/CodeGenPrepare/RISCV/convert-to-eqz.ll

@@ -0,0 +1,80 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -codegenprepare -S -mtriple=riscv64 < %s | FileCheck %s
+
+define i8 @hoist_add(i8 %x) {
+; CHECK-LABEL: define i8 @hoist_add(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INC:%.*]] = add i8 [[X]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[EXIT:%.*]], label [[IF_THEN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i8 [ [[INC]], [[IF_THEN]] ], [ -1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i8 [[RETVAL]]
+;
+entry:
+  %cmp = icmp eq i8 %x, -1
+  br i1 %cmp, label %exit, label %if.then
+
+if.then:
+  %inc = add nuw nsw i8 %x, 1
+  br label %exit
+
+exit:
+  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
+  ret i8 %retval
+}
+
+define i8 @hoist_lshr(i8 %x) {
+; CHECK-LABEL: define i8 @hoist_lshr(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INC:%.*]] = lshr i8 [[X]], 3
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[EXIT:%.*]], label [[IF_THEN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i8 [ [[INC]], [[IF_THEN]] ], [ -1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i8 [[RETVAL]]
+;
+entry:
+  %cmp = icmp ult i8 %x, 8
+  br i1 %cmp, label %exit, label %if.then
+
+if.then:
+  %inc = lshr exact i8 %x, 3
+  br label %exit
+
+exit:
+  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
+  ret i8 %retval
+}
+
+define i8 @nomove_add(i8 %x) {
+; CHECK-LABEL: define i8 @nomove_add(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INC:%.*]] = add i8 [[X]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[EXIT:%.*]], label [[IF_THEN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i8 [ [[INC]], [[IF_THEN]] ], [ -1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i8 [[RETVAL]]
+;
+entry:
+  %inc = add nuw nsw i8 %x, 1
+  %cmp = icmp eq i8 %x, -1
+  br i1 %cmp, label %exit, label %if.then
+
+if.then:
+  br label %exit
+
+exit:
+  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
+  ret i8 %retval
+}