[AMDGPU] Shrink to SOPK with 32-bit signed literals (#70263)

A literal like 0xffff8000 is valid to be used as KIMM in a SOPK
instruction, but at the moment our checks expect it to be fully sign
extended to a 64-bit signed integer. This is not required since all
cases which are being shrunk only accept 32-bit operands.

We need to sign extend the operand to 64-bit though so it passes the
verifier and properly printed.