r600/sfn: Add lowering UBO access to r600 specific codes

r600/sfn: Add lowering UBO access to r600 specific codes

r600 reads vec4 from the UBO, but the offsets in nir are evaluated to the component.
If the offsets are not literal then all non-vec4 reads must resolve the component
after reading a vec4 component (TODO: figure out whether there is a consistent way
to deduct the component that is actually read).

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3225>