AMDGPU: Handle s_add_u32 in eliminateFrameIndex (#129628)
We can fold frame indexes directly into existing immediate operands,
just like is already done for s_add_i32. We happen to use s_add_i32 in
the 32-bit add case, but s_add_u32 appears in the a 64-bit add sequence
of a flat pointer if an addrpacecast source is a frame index.
This avoids, but does not address a failure exposed after
a3165398db0736588daedb07650195502592e567 where two literal operands
end up in the final instruction. The underlying issue still exists for
some instructions without special handling in eliminateFrameIndex.
