intel/fs: Introduce barycentric layout lowering pass.

intel/fs: Introduce barycentric layout lowering pass.

The goal is to represent barycentrics with the standard vector layout
during optimization and particularly SIMD lowering.  Instead of
emitting the barycentric layout conversions at NIR translation time,
do it later as a lowering pass.  For the moment this is only applied
to PI messages, but we'll give the same treatment to LINTERP
instructions too.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>