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

[Yingwei Zheng via llvm-commits]

================
@@ -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) {
+    Value *BestValue = nullptr;
+    Value *NullValue = Constant::getNullValue(Ty);
     for (const auto *U : I.users()) {
-      Constant *C = NullValue;
+      Value *C = NullValue;
       if (match(U, m_Or(m_Value(), m_Value())))
         C = ConstantInt::getAllOnesValue(Ty);
       else if (match(U, m_Select(m_Specific(&I), m_Constant(), m_Value())))
         C = ConstantInt::getTrue(Ty);
+      else if (match(&I, m_Freeze(m_Poison())) &&
+               match(U, m_c_Select(m_Specific(&I), m_Value(C)))) {
+        if (!isGuaranteedNotToBeUndefOrPoison(C, &AC, &I, &DT))
----------------
dtcxzyw wrote:

See the replacement selection logic below:
```
if (!BestValue)
  BestValue = C;
else if (BestValue != C)
  BestValue = NullValue;
```
It guarantees that all the replacements are the same.

This is what I have in mind:
```
auto getScalarUndefReplacement = ...;
Constant *C;
if (match(Op0, m_Constant(C)) && C->containsUndefOrPoisonElement()) {
  Constant *ReplaceC = getScalarUndefReplacement(I.getType()->getScalarType());
  return replaceInstUsesWith(I, Constant::replaceUndefsWith(C, ReplaceC));
}

auto getUndefReplacement = [&I] {
    Value *BestValue = nullptr;
    for (const auto *U : I.users()) {
      Value *C = nullptr;
      if (!(match(&I, m_Freeze(m_Poison())) &&
               match(U, m_c_Select(m_Specific(&I), m_Value(C))))
        return nullptr;

      if (!BestValue)
        BestValue = C;
      else if (BestValue != C)
        return nullptr;
    }
    return BestValue;
  };
if (auto *Rep = getUndefReplacement())
  if (isGuaranteedNotToBeUndefOrPoison(Rep))
    return replaceInstUsesWith(I, Rep);
```


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