i965/vec4: if register allocation fails, don't try to schedule.

Otherwise the scheduler would be invoked with prog_data->total_grf ==
0, causing havoc.

In a future patch, this will allow us to try compiling a geometry
shader in DUAL_OBJECT mode with spilling disabled, and then fall back
to DUAL_INSTANCED mode if that failed.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_vec4.cpp

@@ -1520,7 +1520,7 @@ vec4_visitor::run()
 
    while (!reg_allocate()) {
       if (failed)
-         break;
+         return false;
    }
 
    opt_schedule_instructions();