[llvm] [VectorCombine] Avoid inserting freeze when scalarizing extend-extract if all extracts would lead to UB on poison. (PR #164683)

[Florian Hahn via llvm-commits]

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@@ -2017,8 +2017,31 @@ bool VectorCombine::scalarizeExtExtract(Instruction &I) {
 
   Value *ScalarV = Ext->getOperand(0);
   if (!isGuaranteedNotToBePoison(ScalarV, &AC, dyn_cast<Instruction>(ScalarV),
-                                 &DT))
-    ScalarV = Builder.CreateFreeze(ScalarV);
+                                 &DT)) {
+    // Check wether all lanes are extracted, all extracts trigger UB on
+    // poison, and the last extract (and hence all previous ones)
+    // are guaranteed to execute if Ext executes.
+    // If so, we do not need to insert a freeze.
----------------
fhahn wrote:

```suggestion
    // are guaranteed to execute if Ext executes.  If so, we do not need to insert a freeze.
```

https://github.com/llvm/llvm-project/pull/164683