[llvm] [LV] Support multiplies by constants when forming scaled reductions. (PR #161092)

[David Sherwood via llvm-commits]

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@@ -340,6 +340,15 @@ VPPartialReductionRecipe::computeCost(ElementCount VF,
                                                  : Widen->getOperand(1));
     ExtAType = GetExtendKind(ExtAR);
     ExtBType = GetExtendKind(ExtBR);
+
+    if (!ExtBR && Widen->getOperand(1)->isLiveIn()) {
+      auto *CI =
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david-arm wrote:

Can you just use `cast<ConstantInt>` here because I don't think we should ever get here for non-constant live-ins, right?

https://github.com/llvm/llvm-project/pull/161092