[llvm] [InstCombine] Enable select freeze poison folding when storing value (PR #129776)

[John McIver via llvm-commits]

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@@ -4812,15 +4812,20 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
   //
   // TODO: This could use getBinopAbsorber() / getBinopIdentity() to avoid
   //       duplicating logic for binops at least.
-  auto getUndefReplacement = [&I](Type *Ty) {
-    Constant *BestValue = nullptr;
-    Constant *NullValue = Constant::getNullValue(Ty);
+  auto getUndefReplacement = [&I, &AC = this->AC, &DT = this->DT](Type *Ty) {
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jmciver wrote:

>From a high level should my implementation plan be?

1. Simplify getUndefReplacement helper to only handle select scalars
2. Implement poisonous constant expressions
3. Implement vector splat generation

https://github.com/llvm/llvm-project/pull/129776