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

It checks for the copy of subregs, but it checks destination which may
never happen in SSA. It misses the subreg check and happily produces
S_MOV_B64 out of a subreg COPY.

The affected test should have never been formed in the first place
because the pass is running in SSA and copies into a subreg shall never
happen.