nir/xfb: add component_offset at nir_xfb_info

nir/xfb: add component_offset at nir_xfb_info

Where component_offset here is the offset when accessing components of
a packed variable. Or in other words, location_frac on
nir.h. Different places of mesa use different names for it.

Technically nir_xfb_info consumer can get the same from the
component_mask, it seems somewhat forced to make it to compute it,
instead of providing it.

v2: rename local location_frac for comp_offset, more similar to the
intended use (Timothy Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>