intel/fs: Set up FB write message headers in the visitor

intel/fs: Set up FB write message headers in the visitor

Doing instruction header setup in the generator is awful for a number
of reasons.  For one, we can't schedule the header setup at all.  For
another, it means lots of implied writes which the instruction scheduler
and other passes can't properly read about.  The second isn't a huge
problem for FB writes since they always happen at the end.  We made a
similar change to sampler handling in ff4726077d86.

Reviewed-by: Matt Turner <mattst88@gmail.com>