intel/compiler: Reverse inclusion dependency between brw_vec4_live_variables.h and...

intel/compiler: Reverse inclusion dependency between brw_vec4_live_variables.h and brw_vec4.h

brw_vec4.h (in particular vec4_visitor) is logically a user of the
live variables analysis pass, not the other way around.
brw_vec4_live_variables.h requires the definition of some VEC4 IR data
structures to compile, but those can be obtained directly from
brw_ir_vec4.h without including brw_vec4.h.

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