[llvm] [AMDGPU] Fix subreg check in the SIFixSGPRCopies (PR #70007)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> > LGTM.
> > > It checks for the copy of subregs, but it checks destination which may never happen in SSA.
> > 
> > 
> > Maybe MachineVerifier should check that.
> 
> Maybe as a global check, that a def cannot have subreg in SSA?

Actually I've seen these checks. But it will not help here with the MIR test, it just does not say if we are in SSA.

https://github.com/llvm/llvm-project/pull/70007