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

[David Sherwood via llvm-commits]

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@@ -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;
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david-arm wrote:

I'm not sure what name I would change it to? Something like `ScaledReductionMap`? `ScalingFactorMap`? 

https://github.com/llvm/llvm-project/pull/123368