[llvm] Reland "[LoopVectorizer] Add support for partial reductions" (PR #120721)

[Florian Hahn via llvm-commits]

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@@ -3367,6 +3427,8 @@ void VPFirstOrderRecurrencePHIRecipe::print(raw_ostream &O, const Twine &Indent,
 void VPReductionPHIRecipe::execute(VPTransformState &State) {
   auto &Builder = State.Builder;
 
+  auto VF = State.VF.divideCoefficientBy(VFScaleFactor);
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fhahn wrote:

This (and a new test) are the only changes to the landed diff, right? Could you add a comment about the scaling here?

Maybe spell out VFs type to make it obvious if it the whole line stays < 80 chars
```suggestion
  ElementCount VF = State.VF.divideCoefficientBy(VFScaleFactor);
```

https://github.com/llvm/llvm-project/pull/120721