AMDGPU: Fix DAG divergence for implicit function arguments

This should be directly implied from the register class, and there's
no need to special case live ins here. This was getting the wrong
answer for the queue ptr argument in callable functions, since it's
not an explicit IR argument and is always uniform.

Fixes not using scalar loads for the aperture in addrspacecast
lowering, and any other places that use implicit SGPR arguments.