i965/vec4: Move vec4 data structures and functions to brw_vec4.{cpp,h}.

i965/vec4: Move vec4 data structures and functions to brw_vec4.{cpp,h}.

This patch moves the following things into brw_vec4.{cpp,h}:

- struct brw_vec4_compile
- struct brw_vec4_prog_key
- brw_vec4_prog_data_compare()
- brw_vec4_prog_data_free()

This will allow us to avoid having to include brw_vs.h in
geometry-shader-specific files.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>