[llvm] AMDGPU: Mark DS instructions as fixed size (PR #156388)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

OK but I wish we had better verification of these size calculations. This check does approximately nothing since it is skipped for pseudos but MI is always a pseudo here: https://github.com/llvm/llvm-project/blob/e867b8511814e75abbbe7048d505f84d4e686961/llvm/lib/Target/AMDGPU/AMDGPUMCInstLower.cpp#L398

https://github.com/llvm/llvm-project/pull/156388