[llvm] [LV] Add support for partial reductions without a binary op (PR #133922)

[Matthew Devereau via llvm-commits]

MDevereau wrote:

Hi, @david-arm asked me to look after this pull request for him.

I've rebased it and have added some new logic to `VPPartialReductionRecipe::computeCost` to handle some situations where different VPlan operands that previously weren't seen are now possible for Partial Reduction recipes.

Regarding the previous conversation about waiting for @SamTebbs33's work, we've discussed the situation internally and would like to prioritize getting this patch along so that it's ready before the branch point for 21.

@fhahn Could you please re-review this patch? Thank you.

https://github.com/llvm/llvm-project/pull/133922