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

[Thorsten Schütt via llvm-commits]

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@@ -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)) {
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tschuett wrote:

Let's go one step back. This discussion is unrelated to this PR. What is it that you don't like about this PR:
* I fixed a bug. We build constants without checking for legality.
* The legality check is too close to the build and you would rather see it 100 miles afar from the build.
* Otherwise, I would need some help to understand the unhappiness.

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