[llvm] [IR][LangRef] Add partial reduction add intrinsic (PR #94499)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> What's the benefit to claiming this is a separate operation from the existing reduction intrinsics? Can't you just loosen the type declaration to allow vector types for the result?

ping? 

https://github.com/llvm/llvm-project/pull/94499