[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Nicholas Guy via llvm-commits]

================
@@ -22,6 +22,26 @@ class LoopVectorizationLegality;
 class LoopVectorizationCostModel;
 class TargetLibraryInfo;
 struct HistogramInfo;
+class TargetTransformInfo;
+
+/// A chain of instructions that form a partial reduction.
+/// Designed to match: reduction_bin_op (bin_op (extend (A), (extend (B))),
+/// accumulator).
+struct PartialReductionChain {
+  PartialReductionChain(Instruction *Reduction, Instruction *ExtendA,
+                        Instruction *ExtendB, Instruction *BinOp)
+      : Reduction(Reduction), ExtendA(ExtendA), ExtendB(ExtendB), BinOp(BinOp) {
+  }
+  /// The top-level binary operation that forms the reduction to a scalar
+  /// after the loop body.
+  Instruction *Reduction;
+  /// The extension of each of the inner binary operation's operands.
+  Instruction *ExtendA;
+  Instruction *ExtendB;
+
+  /// The binary operation using the extends that is then reduced
----------------
NickGuy-Arm wrote:

```suggestion
  /// The binary operation using the extends that is then reduced.
```

https://github.com/llvm/llvm-project/pull/92418