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

Using MMRAs, allow `builtin_amdgcn_fence` to emit fences that only
target one or more address spaces, instead of fencing all address spaces
at once.

This is done through a `amdgpu-as` MMRA. Currently focused on OpenCL
fences, but can very easily support more AS names and codegen on more
than just fences.