intel/compiler: Reverse inclusion dependency between brw_fs_live_variables.h and...

intel/compiler: Reverse inclusion dependency between brw_fs_live_variables.h and brw_fs.h

brw_fs.h (in particular fs_visitor) is logically a user of the live
variables analysis pass, not the other way around.
brw_fs_live_variables.h requires the definition of some FS IR data
structures to compile, but those can be obtained directly from
brw_ir_fs.h without including brw_fs.h.  The dependency of
fs_live_variables on fs_visitor is rather accidental and will be
removed in a future commit, a forward declaration is enough for the
moment.

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