[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

> Thanks for the latest updates! A few more mostly minor comments inline.
> 
> It looks like the current patch needs to be updated on top of current `main`, would be good to do so for a final look. I don't expect many conflicts/issues, but IIRC ::computeCost now needs to be provided.

I'll rebase now, thanks for the review!

https://github.com/llvm/llvm-project/pull/92418