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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> > I thought image memory = private. It's unclear to me, what AS does OpenCL IMAGE memory map to in our backend? (But otherwise, yes, MMRA should just have the backend names, the mapping of the OpenCL IMAGE to a backend AS should be in the device-lib)
> 
> Images are global memory with magical addressing and value interpretation on load/store. There's nothing private about them

So in our case:

- OpenCL Image = Global
- OpenCL Local = LDS
- OpenCL global = Global 
?

Is that correct?

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?

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