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

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

I've folded the target hook code into the AArch64 lowering and created a PR for that [here](https://github.com/llvm/llvm-project/pull/101010).

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