[AMDGPU] Fix lowering of @llvm.amdgcn.set.inactive(imm, poison)

If the second argument of V_SET_INACTIVE is undef/poison,
SIWholeQuadMode lowered it to a COPY from the first argument, but that
caused invalid MIR if the first argument was an immediate rather than a
register.

Fix this by lowering to a V_MOV instruction instead of a COPY.

Fixes https://github.com/llvm/llvm-project/issues/62862

Differential Revision: https://reviews.llvm.org/D151105