[AMDGPU] Fix copyPhysReg to not produce unalined vgpr access

RA can insert something like a sub1_sub2 COPY of a wide VGPR
tuple which results in the unaligned acces with v_pk_mov_b32
after the copy is expanded. This is regression after D97316.

Differential Revision: https://reviews.llvm.org/D98549