unboxing improvement in the JIT

Improve the "can unbox test" to include patterns that were already covered
by the more restrictive "can unbox without disturbing N registers" test
(which is always tried first, but the weaker test recurs within patterns
that the stronger test does not recognize).

racket/src/racket/src/jitarith.c

@@ -360,7 +360,7 @@ int can_unbox_directly(Scheme_Object *obj, int extfl, int bfuel)
       obj = ((Scheme_Sequence *)obj)->array[((Scheme_Sequence *)obj)->count - 1];
       break;
     default:
-      return 0;
+      return is_unboxing_immediate(obj, 0, extfl);
     }
   }
 }