[clang] [llvm] [AMDGPU] Add amdgpu-as MMRA for fences (PR #78572)

[Pierre van Houtryve via cfe-commits]

Pierre-vh wrote:

> > Then I guess the MMRA should just have "global" and "local" for now, we can always add more later if needed. What do you think?
> 
> Yes, we don't have specific image counters. They are just vcmnt

Diff has been updated with those changes

https://github.com/llvm/llvm-project/pull/78572