[llvm] [LangRef] Rewrite the elementwise atomic description and reject seq_cst elementwise atomics (PR #209931)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

> It is unfortunate to have submitted elementwise load already, because this change must also be cherry-picked back into the 23.x release branch -- which'll be harder to do when it's rebased on top of that.

Do we need to split the atomic load changes into a separate PR to make the backport to the 23.x release branch easier?

https://github.com/llvm/llvm-project/pull/209931