[llvm] [LV] Replace "BinOp" with "ExtendedOp" in partial reduction transforms (NFCI) (PR #192422)

[Sander de Smalen via llvm-commits]

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@@ -6004,57 +6004,62 @@ struct ExtendedReductionOperand {
 };
 
 /// A chain of recipes that form a partial reduction. Matches either
-///   reduction_bin_op (extend (A), accumulator), or
-///   reduction_bin_op (bin_op (extend (A), (extend (B))), accumulator).
+///   reduction_bin_op (extended op, accumulator), or
+///   reduction_bin_op (accumulator, extended op).
+/// The possible forms of the "extended op" are listed in
+/// matchExtendedReductionOperand.
 struct VPPartialReductionChain {
   /// The top-level binary operation that forms the reduction to a scalar
   /// after the loop body.
   VPWidenRecipe *ReductionBinOp = nullptr;
   /// The user of the extends that is then reduced.
   ExtendedReductionOperand ExtendedOp;
-  unsigned ScaleFactor;
   /// The recurrence kind for the entire partial reduction chain.
   /// This allows distinguishing between Sub and AddWithSub recurrences,
   /// when the ReductionBinOp is a Instruction::Sub.
   RecurKind RK;
+  /// The index of the accumulator operand of ReductionBinOp. The extended op
+  /// is `1 - AccumulatorOpIdx`.
+  unsigned AccumulatorOpIdx;
+  unsigned ScaleFactor;
 };
 
 static VPSingleDefRecipe *
-optimizeExtendsForPartialReduction(VPSingleDefRecipe *BinOp,
+optimizeExtendsForPartialReduction(VPSingleDefRecipe *ExtendedOp,
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sdesmalen-arm wrote:

nit: maybe just call this `Op` ?

https://github.com/llvm/llvm-project/pull/192422