AMDGPU: Inline constant when materalizing FI with add on gfx9

This was relying on the SGPR usable for the carry out clobber to also
be used for the input. There was no carry out on gfx9. With no carry
out clobber to worry about, so the literal can just be directly used
with a VOP2 add.

llvm-svn: 371791