[llvm] [ConstantFolding] Fold scalable shufflevector of poison/undef. (PR #143475)

[Nikita Popov via llvm-commits]

================
@@ -463,10 +463,7 @@ Constant *llvm::ConstantFoldShuffleVectorInstruction(Constant *V1, Constant *V2,
     Constant *Elt =
         ConstantExpr::getExtractElement(V1, ConstantInt::get(Ty, 0));
 
-    if (Elt->isNullValue()) {
-      auto *VTy = VectorType::get(EltTy, MaskEltCount);
-      return ConstantAggregateZero::get(VTy);
-    } else if (!MaskEltCount.isScalable())
+    if (!MaskEltCount.isScalable() || Elt->isNullValue() || isa<UndefValue>(Elt))
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nikic wrote:

It took me a while to get why we need this condition. I'd add a comment like
```
// For scalable vectors, make sure this doesn't fold back into a shufflevector.
```

https://github.com/llvm/llvm-project/pull/143475