nir/deref: Add helpers for getting offsets

nir/deref: Add helpers for getting offsets

These are very similar to the related function in nir_lower_io except
that they don't handle per-vertex or packed things (that could be added,
in theory) and they take a more detailed size/align function pointer.
One day, we should consider switching nir_lower_io over to using the
more detailed size/align functions and then we could make it use these
helpers instead of having its own.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>