AMDGPU: Relax restriction on folding immediates into physregs

I never completed the work on the patches referenced by
f8bf7d7f, but this was intended to
avoid folding immediate writes into m0 which the coalescer doesn't
understand very well. Relax this to allow simple SGPR immediates to
fold directly into VGPR copies. This pattern shows up routinely in
current GlobalISel code since nothing is smart enough to emit VGPR
constants yet.