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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> Was there an RFC for this anywhere yet to point to the review? The reviewer list looks a bit narrow. Thanks

There is https://discourse.llvm.org/t/rfc-is-a-more-expressive-way-to-represent-reductions-useful/74929, which started the conversation but as you'll see nobody seemed interested and so I said Arm would take the lead.  We should have referenced the PR when available though, which I've rectified.

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