i965/fs: Add a shader_stats struct.

It'll grow further, and we'd like to avoid adding an additional
parameter to fs_generator() for each new piece of data.

v2 (idr): Rebase on 17 months.  Track a visitor instead of a cfg.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>