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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

AFAICT this isn't driven by cost at all? Could this be done as VPlan-to-VPlan transform that replaces regular reduction recipes with partial ones?

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