[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?

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