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

[Sam Tebbs via llvm-commits]

https://github.com/SamTebbs33 commented:

I've rebased to address the merge conflicts. I've also changed the cost assertion to a return, since the partial reduction variant with a constant being multiplied was receiving an invalid cost from `getPartialReductionCost` and failed the assertion. I think it's best to do the work to bundle partial reductions with constants separately since this PR is large enough as it is.

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