[llvm] [VPlan] Create header phis once, after constructing VPlan0 (NFC). (PR #168291)

[Florian Hahn via llvm-commits]

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@@ -8357,6 +8265,9 @@ VPRecipeBuilder::tryToCreatePartialReduction(VPInstruction *Reduction,
        cast<VPReductionRecipe>(BinOpRecipe)->isPartialReduction()))
     std::swap(BinOp, Accumulator);
 
+  if (auto *RedPhiR = dyn_cast<VPReductionPHIRecipe>(Accumulator))
+    RedPhiR->setVFScaleFactor(ScaleFactor);
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fhahn wrote:

Yep it could be done separately, but only needed as part of this change

https://github.com/llvm/llvm-project/pull/168291