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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> > It looks like `LLVM.CodeGen/WebAssembly/partial-reduce-accumulate.ll` is crashing with the change
> 
> Yep, it looks the wrong operand index was being used in `VPPartialReductionRecipe::computeCost` and it's been hidden until now. Fixed.

Could you add a dedicated LV test for that to make sure we have dedicated test coverage w/o relying on a codegen test that runs LV?

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