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

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

LGTM.

> It checks for the copy of subregs, but it checks destination which may never happen in SSA.

Maybe MachineVerifier should check that.

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