[AMDGPU] Implement moveToVALU for S_CSELECT_B64 (#70352)

moveToVALU previously only handled S_CSELECT_B64 in the trivial case
where it was semantically equivalent to a copy. Implement the general
case using V_CNDMASK_B64_PSEUDO and implement post-RA expansion of
V_CNDMASK_B64_PSEUDO with immediate as well as register operands.