Define PREV_CLASS_SERIAL

Avoids genereating a "throwaway" sentinel class serial. There wasn't any
read harm in doing so (we're at no risk of exhaustion and there'd be no
measurable performance impact), but if feels cleaner that all class
serials actually end up assigned and used (especially now that we won't
overwrite them in a single method definition).

vm_insnhelper.h

@@ -184,6 +184,7 @@ CC_SET_ME(CALL_CACHE cc, const rb_callable_method_entry_t *me)
 } while (0)
 #endif
 
+#define PREV_CLASS_SERIAL() (ruby_vm_class_serial)
 #define NEXT_CLASS_SERIAL() (++ruby_vm_class_serial)
 #define GET_GLOBAL_METHOD_STATE() (ruby_vm_global_method_state)
 #define INC_GLOBAL_METHOD_STATE() (++ruby_vm_global_method_state)

vm_method.c

@@ -104,7 +104,7 @@ rb_clear_method_cache_by_class(VALUE klass)
 	    INC_GLOBAL_METHOD_STATE();
 	}
 	else {
-	    rb_serial_t old_serial = rb_next_class_serial();
+	    rb_serial_t old_serial = PREV_CLASS_SERIAL();
 	    rb_class_clear_method_cache(klass, (VALUE)&old_serial);
 	}
     }