[llvm] [AMDGPU][MC] Restrict op_sel in VOP3P dot instructions (PR #100485)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> > I would disallow it. Why pre-GFX11 only? It does not do anything for dot4 and dot8 on any target.
> 
> The reason for limiting this to pre-GFX11 is that Sect 7.5.1 of the RDNA3 ISA doc indicates "ignore OPSEL" for dot4_* and dot8_*, which seems to imply it's allowed. But I'm fine with disallowing it for all targets, which is actually what the 1st commit did.

Technically you can encode it, but it does not affect the execution. No reason to handle it differently between targets, just set encoding bits to default.

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