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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Seems reasonable. Is this NFC? Could also do this on pretty much everything except SOP* and VOP*.

Yes. It happened to bypass a crash I ran into on a later patch, and replaced it with a friendlier error later 

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