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>

src/intel/compiler/brw_vec4_live_variables.cpp

@@ -25,7 +25,7 @@
  *
  */
 
-#include "brw_cfg.h"
+#include "brw_vec4.h"
 #include "brw_vec4_live_variables.h"
 
 using namespace brw;