[llvm] [InstCombine] Opt phi(freeze(undef), C) -> phi(C, C) (PR #161181)

[Yingwei Zheng via llvm-commits]

================
@@ -5179,9 +5180,35 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
   // TODO: This could use getBinopAbsorber() / getBinopIdentity() to avoid
   //       duplicating logic for binops at least.
   auto getUndefReplacement = [&](Type *Ty) {
-    Value *BestValue = nullptr;
+    auto pickCommonConstantFromPHI = [&](PHINode &PN) -> Value * {
+      // phi(freeze(undef), C, C). Choose C for freeze so the PHI can be
+      // removed.
+      Constant *BestValue = nullptr;
+      Constant *C = nullptr;
+      for (Value *V : PN.incoming_values()) {
+        if (match(V, m_Freeze(m_Undef())))
+          continue;
+
+        if (!isa<Constant>(V))
+          return nullptr;
+
+        if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
----------------
dtcxzyw wrote:

```suggestion
        if (!isGuaranteedNotToBeUndefOrPoison(V))
```
Context information is unused for constants.

https://github.com/llvm/llvm-project/pull/161181