[AMDGPU] Lower VGPR to physical SGPR COPY to S_MOV_B32 if VGPR contains the compile time constant

Sometimes we have a constant value loaded to VGPR. In case we further
need to rematrerialize it in the physical scalar register we may avoid VGPR to
SGPR copy replacing it with S_MOV_B32.

Reviewed By: JonChesterfield, arsenm

Differential Revision: https://reviews.llvm.org/D139874