[VPlan] Set VF scale factor in tryToCreatePartialReduction (NFCI).

Split off unrelated change from approved
#168291 to land separately as
suggested.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8247,8 +8247,8 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
       // If the PHI is used by a partial reduction, set the scale factor.
       bool UseInLoopReduction = CM.isInLoopReduction(Phi);
       bool UseOrderedReductions = CM.useOrderedReductions(RdxDesc);
-      unsigned ScaleFactor =
-          getScalingForReduction(RdxDesc.getLoopExitInstr()).value_or(1);
+      // Will be updated later to >1 if reduction is partial.
+      unsigned ScaleFactor = 1;
 
       return new VPReductionPHIRecipe(
           Phi, RdxDesc.getRecurrenceKind(), *StartV, *BackedgeValue,
@@ -8327,6 +8327,9 @@ VPRecipeBuilder::tryToCreatePartialReduction(VPInstruction *Reduction,
        cast<VPReductionRecipe>(BinOpRecipe)->isPartialReduction()))
     std::swap(BinOp, Accumulator);
 
+  if (auto *RedPhiR = dyn_cast<VPReductionPHIRecipe>(Accumulator))
+    RedPhiR->setVFScaleFactor(ScaleFactor);
+
   assert(ScaleFactor ==
              vputils::getVFScaleFactor(Accumulator->getDefiningRecipe()) &&
          "all accumulators in chain must have same scale factor");

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2474,6 +2474,13 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
     return Partial ? Partial->VFScaleFactor : 1;
   }
 
+  /// Set the VFScaleFactor for this reduction phi. Can only be set to a factor
+  /// > 1.
+  void setVFScaleFactor(unsigned ScaleFactor) {
+    assert(ScaleFactor > 1 && "must set to scale factor > 1");
+    Style = RdxUnordered{ScaleFactor};
+  }
+
   /// Returns the number of incoming values, also number of incoming blocks.
   /// Note that at the moment, VPWidenPointerInductionRecipe only has a single
   /// incoming value, its start value.