[llvm] [GlobalISel] Combine G_ZEXT of undef -> 0 (PR #113764)

[Thorsten Schütt via llvm-commits]

================
@@ -2916,8 +2916,11 @@ void CombinerHelper::replaceInstWithFConstant(MachineInstr &MI, double C) {
 
 void CombinerHelper::replaceInstWithConstant(MachineInstr &MI, int64_t C) {
   assert(MI.getNumDefs() == 1 && "Expected only one def?");
-  Builder.buildConstant(MI.getOperand(0), C);
-  MI.eraseFromParent();
+  LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
+  if (isConstantLegalOrBeforeLegalizer(DstTy)) {
----------------
tschuett wrote:

It depends on where this code is running.
```
void CombinerHelper::replaceInstWithConstant(MachineInstr &MI, int64_t C) {
  assert(MI.getNumDefs() == 1 && "Expected only one def?");
  
  LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
  Register SrcReg = MI.getOperand(1).getReg();
  if (isConstantLegalOrBeforeLegalizer(DstTy) && MRI.hasOneNonDbgUse(SrcReg)) {
    Builder.buildConstant(MI.getOperand(0), C);
    MI.eraseFromParent();
  }
}
```

https://github.com/llvm/llvm-project/pull/113764