[llvm] [AMDGPU] Document amdgpu-as in AMDGPUUsage (PR #94335)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

Thanks. I think this looks good.

As noted offline, I do think we should also accept `amdgpu-as` on atomic operations for consistency, but that's something for a different change.

https://github.com/llvm/llvm-project/pull/94335