[llvm] [AMDGPU] Add support for store to constant address space (PR #153835)

[Shilei Tian via llvm-commits]

shiltian wrote:

This is marked as WIP at this moment because I have not done the GISel part. I'm not sure if the combiner is the right way to do this, but I don't think we can and should do this in the pattern matching as well. On the other hand, if this approach is fine, I'll also do the atomic.

https://github.com/llvm/llvm-project/pull/153835