[llvm] [LV] Use VPReductionRecipe for partial reductions (PR #144908)

[Sam Tebbs via llvm-commits]

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@@ -2408,23 +2414,32 @@ class VPInterleaveRecipe : public VPRecipeBase {
   Instruction *getInsertPos() const { return IG->getInsertPos(); }
 };
 
-/// A recipe to represent inloop reduction operations, performing a reduction on
-/// a vector operand into a scalar value, and adding the result to a chain.
-/// The Operands are {ChainOp, VecOp, [Condition]}.
+/// A recipe to represent inloop, ordered or partial reduction operations. It
+/// performs a reduction on a vector operand into a scalar (vector in the case
+/// of a partial reduction) value, and adds the result to a chain. The Operands
+/// are {ChainOp, VecOp, [Condition]}.
 class VPReductionRecipe : public VPRecipeWithIRFlags {
   /// The recurrence kind for the reduction in question.
   RecurKind RdxKind;
   bool IsOrdered;
   /// Whether the reduction is conditional.
   bool IsConditional = false;
+  /// The scaling factor, relative to the VF, that this recipe's output is
+  /// divided by.
+  /// For outer-loop reductions this is equal to 1.
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SamTebbs33 wrote:

Yeah bundling comes mostly for free if partial reductions are just reductions, and otherwise, it doesn't really make sense to differentiate them considering that the only difference is the scale factor being > 1. I think lowering complexity by reducing the number of different recipes in existence is worth the small bit of extra complexity in the reduction class. 

https://github.com/llvm/llvm-project/pull/144908