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

[via llvm-commits]

NickGuy-Arm wrote:

I've separated out the recent work into logical chunks that, while conceptually could be separate PRs, are still somewhat inter-dependent and are untested in isolation. I could separate them out to different PRs if necessary, however I feel there is value in not fragmenting any discussions.

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