[llvm] [VPlan] Narrow VPWidenCastRecipe to scalar cast recipe. (PR #166514)

[Luke Lau via llvm-commits]

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@@ -171,6 +171,10 @@ bool vputils::isSingleScalar(const VPValue *VPV) {
     return PreservesUniformity(WidenR->getOpcode()) &&
            all_of(WidenR->operands(), isSingleScalar);
   }
+  if (auto *CastR = dyn_cast<VPWidenCastRecipe>(VPV)) {
+    return PreservesUniformity(CastR->getOpcode()) &&
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lukel97 wrote:

Copying over my review from #143552 , I think this PreservesUniformity call is always true, we can remove it

https://github.com/llvm/llvm-project/pull/166514