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

[via llvm-commits]

NickGuy-Arm wrote:

This patch only implements the pattern recognition and production of the partial reduction intrinsic, it does not yet lower the intrinsic to valid IR/Asm, those will be coming later.
I'm also away for the next week, so will address comments when I return

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