[llvm] [LV] NFCI: Create VPExpressions in transformToPartialReductions. (PR #182863)

[Sander de Smalen via llvm-commits]

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@@ -6130,6 +6148,12 @@ static void transformToPartialReduction(const VPPartialReductionChain &Chain,
     ExitValue->replaceAllUsesWith(PartialRed);
   WidenRecipe->replaceAllUsesWith(PartialRed);
 
+  // For cost-model purposes, see if we can fold this into a VPExpression.
+  if (VPExpressionRecipe *E = createPartialReductionExpression(PartialRed)) {
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sdesmalen-arm wrote:

We would still be able to form partial reductions, it's just that the cost-model wouldn't be entirely accurate because it wouldn't use a VPExpression, but rather cost each node of that expression individually.

I've asserted it now, and also limited `matchExtendedReductonOperand` to only support mul/fmul.

https://github.com/llvm/llvm-project/pull/182863