llvm-reduce: Don't set generic instruction operands to undef

The intention is that these should never have undef operands. It turns
out the restriction the verifier enforces is too lax. The verifier
enforces that registers without a register class cannot be undef, but
it's valid to use a register with a register class and type. The
verifier needs to change to be based on the opcode.

llvm/test/tools/llvm-reduce/mir/generic-vreg.mir

@@ -1,5 +1,5 @@
 # REQUIRES: amdgpu-registered-target
-# RUN: llvm-reduce -mtriple=amdgcn-amd-amdhsa --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t 2> %t.log
+# RUN: llvm-reduce -abort-on-invalid-reduction -mtriple=amdgcn-amd-amdhsa --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t 2> %t.log
 # RUN: FileCheck --match-full-lines --check-prefix=RESULT %s < %t
 
 # Verify that reduction works with generic virtual registers, and the

llvm/test/tools/llvm-reduce/mir/reduce-register-uses-generic.mir

@@ -0,0 +1,25 @@
+# REQUIRES: amdgpu-registered-target
+# RUN: llvm-reduce -abort-on-invalid-reduction -simplify-mir --delta-passes=register-uses -mtriple=amdgcn-amd-amdhsa --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t 2> %t.log
+# RUN: FileCheck --match-full-lines --check-prefix=RESULT %s < %t
+
+# Generic instructions should not have undef set on operands
+# CHECK-INTERESTINGNESS: G_ADD
+
+# RESULT: %1:vreg_64(s64) = IMPLICIT_DEF
+# RESULT: %add:_(s64) = G_ADD %1, %1
+
+---
+name: func
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1, $vgpr2_vgpr3
+
+    %0:vgpr(s32) = G_IMPLICIT_DEF
+    %1:vreg_64(s64) = IMPLICIT_DEF
+    %add:_(s64) = G_ADD %1, %1
+    %ptr:_(p1) = G_IMPLICIT_DEF
+    G_STORE %0(s32), %ptr(p1) :: (store (s32), addrspace 1)
+    S_ENDPGM 0, implicit %add(s64), implicit %1(s64)
+
+...

llvm/tools/llvm-reduce/deltas/ReduceRegisterUses.cpp

@@ -22,6 +22,10 @@ static void removeUsesFromFunction(Oracle &O, MachineFunction &MF) {
 
   for (MachineBasicBlock &MBB : MF) {
     for (MachineInstr &MI : MBB) {
+      // Generic instructions are not supposed to have undef operands.
+      if (isPreISelGenericOpcode(MI.getOpcode()))
+        continue;
+
       int NumOperands = MI.getNumOperands();
       int NumRequiredOps = MI.getNumExplicitOperands() +
                            MI.getDesc().getNumImplicitDefs() +