[LoopVectorize][NFC] Simplify ScaledReductionExitInstrs map

For the following variable

  DenseMap<const Instruction *, std::pair<PartialReductionChain, unsigned>>
  ScaledReductionExitInstrs;

we never actually need the PartialReductionChain when using the
map. I've cleaned this up so that this now becomes

  DenseMap<const Instruction *, unsigned> ScaledReductionExitInstrs;

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8711,7 +8711,8 @@ void VPRecipeBuilder::collectScaledReductions(VFRange &Range) {
     PartialReductionChain Chain = Pair.first;
     if (ExtendIsOnlyUsedByPartialReductions(Chain.ExtendA) &&
         ExtendIsOnlyUsedByPartialReductions(Chain.ExtendB))
-      ScaledReductionExitInstrs.insert(std::make_pair(Chain.Reduction, Pair));
+      ScaledReductionExitInstrs.insert(
+          std::make_pair(Chain.Reduction, Pair.second));
   }
 }
 
@@ -8803,9 +8804,9 @@ VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
              Phi->getIncomingValueForBlock(OrigLoop->getLoopPreheader()));
 
       // If the PHI is used by a partial reduction, set the scale factor.
-      std::optional<std::pair<PartialReductionChain, unsigned>> Pair =
+      std::optional<unsigned> Scale =
           getScaledReductionForInstr(RdxDesc.getLoopExitInstr());
-      unsigned ScaleFactor = Pair ? Pair->second : 1;
+      unsigned ScaleFactor = Scale ? *Scale : 1;
       PhiRecipe = new VPReductionPHIRecipe(
           Phi, RdxDesc, *StartV, CM.isInLoopReduction(Phi),
           CM.useOrderedReductions(RdxDesc), ScaleFactor);

llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h

@@ -88,8 +88,7 @@ class VPRecipeBuilder {
 
   /// The set of reduction exit instructions that will be scaled to
   /// a smaller VF via partial reductions, paired with the scaling factor.
-  DenseMap<const Instruction *, std::pair<PartialReductionChain, unsigned>>
-      ScaledReductionExitInstrs;
+  DenseMap<const Instruction *, unsigned> ScaledReductionExitInstrs;
 
   /// Check if \p I can be widened at the start of \p Range and possibly
   /// decrease the range such that the returned value holds for the entire \p
@@ -157,7 +156,7 @@ class VPRecipeBuilder {
       : Plan(Plan), OrigLoop(OrigLoop), TLI(TLI), TTI(TTI), Legal(Legal),
         CM(CM), PSE(PSE), Builder(Builder) {}
 
-  std::optional<std::pair<PartialReductionChain, unsigned>>
+  std::optional<unsigned>
   getScaledReductionForInstr(const Instruction *ExitInst) {
     auto It = ScaledReductionExitInstrs.find(ExitInst);
     return It == ScaledReductionExitInstrs.end()