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>

src/intel/compiler/brw_fs.h

@@ -167,6 +167,7 @@ public:
    bool lower_integer_multiplication();
    bool lower_minmax();
    bool lower_simd_width();
+   bool lower_barycentrics();
    bool lower_scoreboard();
    bool opt_combine_constants();