[llvm] [LV] Bundle partial reductions inside VPExpressionRecipe (PR #147302)

[Sam Tebbs via llvm-commits]

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@@ -2856,6 +2867,19 @@ InstructionCost VPExpressionRecipe::computeCost(ElementCount VF,
     Opcode = Instruction::Sub;
     LLVM_FALLTHROUGH;
   case ExpressionTypes::ExtMulAccReduction: {
+    if (isa<VPPartialReductionRecipe>(ExpressionRecipes.back())) {
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SamTebbs33 wrote:

Yeah you're right in saying there is some implicit support at the moment, but it never actually happens because of the incorrect costing and because [these](https://github.com/llvm/llvm-project/pull/147302/files#r2215489617) changes weren't added originally.

https://github.com/llvm/llvm-project/pull/147302