[llvm] [AMDGPU][MC] Disallow op_sel in v_dot4 and v_dot8 with 4- or 8-bit packed data (PR #100485)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I don't see any docs on what the semantics of opsel on those instructions would be.

I don't either. I only found a note that says "is no longer supported on DOT2/4/8" for 940. That implies it was previously supported 

https://github.com/llvm/llvm-project/pull/100485