[AMDGPU] Fix SGPR checks in S_MOV_B64_IMM_PSEUDO generation.

The function to generate S_MOV_B64_IMM_PSEUDO was recently modified to
optimize AGPR to AGPR copy but it missed checking for the SGPR
clobbering for the S_MOV_B64_IMM_PSEUDO generation.

Differential Revision: https://reviews.llvm.org/D113005