intel/compiler: Move base IR definitions into a separate header file

intel/compiler: Move base IR definitions into a separate header file

This pulls out the i965 IR definitions into a separate file and leaves
the top-level backend_shader structure and back-end compiler entry
points in brw_shader.h.  The purpose is to keep things tidy and
prevent a nasty circular dependency between brw_cfg.h and
brw_shader.h.  The logical dependency between these data structures
looks like:

 backend_shader (brw_shader.h) -> cfg_t (brw_cfg.h)
    -> bblock_t (brw_cfg.h) -> backend_instruction (brw_shader.h)

This circular header dependency is currently resolved by using forward
declarations of cfg_t/bblock_t in brw_shader.h and having brw_cfg.h
include brw_shader.h, which seems backwards and won't work at all when
the forward declarations of cfg_t/bblock_t are no longer sufficient in
a future commit.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4012>