[clang] [llvm] [AMDGPU] Emit a waitcnt instruction after each memory instruction (PR #79236)

[Jay Foad via cfe-commits]

https://github.com/jayfoad requested changes to this pull request.

I've added _some_ inline comments, but really I don't want to spend the time to review this properly (or maintain it, or extend it for new architectures in future). All this logic already exists in SIInsertWaitcnts. Duplicating it here is not a good design.

https://github.com/llvm/llvm-project/pull/79236