[LoopVectorize][NFC] Simplify ScaledReductionExitInstrs map (llvm#123368)

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> ScaledReductionMap;

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8711,7 +8711,7 @@ void VPRecipeBuilder::collectScaledReductions(VFRange &Range) {
     PartialReductionChain Chain = Pair.first;
     if (ExtendIsOnlyUsedByPartialReductions(Chain.ExtendA) &&
         ExtendIsOnlyUsedByPartialReductions(Chain.ExtendB))
-      ScaledReductionExitInstrs.insert(std::make_pair(Chain.Reduction, Pair));
+      ScaledReductionMap.insert(std::make_pair(Chain.Reduction, Pair.second));
   }
 }
 
@@ -8803,9 +8803,8 @@ 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 =
-          getScaledReductionForInstr(RdxDesc.getLoopExitInstr());
-      unsigned ScaleFactor = Pair ? Pair->second : 1;
+      unsigned ScaleFactor =
+          getScalingForReduction(RdxDesc.getLoopExitInstr()).value_or(1);
       PhiRecipe = new VPReductionPHIRecipe(
           Phi, RdxDesc, *StartV, CM.isInLoopReduction(Phi),
           CM.useOrderedReductions(RdxDesc), ScaleFactor);
@@ -8840,7 +8839,7 @@ VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
   if (isa<LoadInst>(Instr) || isa<StoreInst>(Instr))
     return tryToWidenMemory(Instr, Operands, Range);
 
-  if (getScaledReductionForInstr(Instr))
+  if (getScalingForReduction(Instr))
     return tryToCreatePartialReduction(Instr, Operands);
 
   if (!shouldWiden(Instr, Range))

llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h

@@ -86,10 +86,8 @@ class VPRecipeBuilder {
   /// created.
   SmallVector<VPHeaderPHIRecipe *, 4> PhisToFix;
 
-  /// 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;
+  /// A mapping of partial reduction exit instructions to their scaling factor.
+  DenseMap<const Instruction *, unsigned> ScaledReductionMap;
 
   /// 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
@@ -157,12 +155,10 @@ class VPRecipeBuilder {
       : Plan(Plan), OrigLoop(OrigLoop), TLI(TLI), TTI(TTI), Legal(Legal),
         CM(CM), PSE(PSE), Builder(Builder) {}
 
-  std::optional<std::pair<PartialReductionChain, unsigned>>
-  getScaledReductionForInstr(const Instruction *ExitInst) {
-    auto It = ScaledReductionExitInstrs.find(ExitInst);
-    return It == ScaledReductionExitInstrs.end()
-               ? std::nullopt
-               : std::make_optional(It->second);
+  std::optional<unsigned> getScalingForReduction(const Instruction *ExitInst) {
+    auto It = ScaledReductionMap.find(ExitInst);
+    return It == ScaledReductionMap.end() ? std::nullopt
+                                          : std::make_optional(It->second);
   }
 
   /// Find all possible partial reductions in the loop and track all of those