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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> I could separate them out to different PRs if necessary, however I feel there is value in not fragmenting any discussions.

As a minimum the intrinsic and its code generation should be broken out into its own PR.  There's never a good reason for code generation and IR optimisation work to be combined because the intrinsic should be able to stand on its own merits.

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