[llvm] [LV] Use VPReductionRecipe for partial reductions (PR #147513)

[Sander de Smalen via llvm-commits]

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@@ -2405,6 +2405,30 @@ struct VPFirstOrderRecurrencePHIRecipe : public VPHeaderPHIRecipe {
 #endif
 };
 
+// Possible variants of a reduction.
+
+// This reduction is ordered and in-loop.
+struct RdxOrdered {};
+// This reduction is in-loop.
+struct RdxInLoop {};
+// This reduction is normal with a >= 1 factor that the vector length is
+// scaled down by.
+struct RdxUnordered {
+  // The factor by which the output is scaled down from the VF.
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sdesmalen-arm wrote:

nit: superfluous comment, given the comment above.

https://github.com/llvm/llvm-project/pull/147513