Avoid revisiting seen nodes clearing method cache

rb_clear_method_cache_by_class calls rb_class_clear_method_cache
recursively on subclasses, where it will bump the class serial and clear
some other data (callable_m_tbl, and some mjit data).

Previously this could end up taking a long time to clear all the classes
if the module was included a few levels deep and especially if there
were multiple paths to it in the dependency tree (ie. a class includes
two modules which both include the same other module) as we end up
revisiting class/iclass/module objects multiple times.

This commit avoids revisiting the same object, by short circuiting when
revisit the same object. We can check this efficiently by comparing the
class serial of each object we visit with the next class serial at the
start. We know that any objects with a higher class serial have already
been visited.

vm_method.c

@@ -62,6 +62,11 @@ static struct {
 static void
 rb_class_clear_method_cache(VALUE klass, VALUE arg)
 {
+    VALUE old_serial = *(rb_serial_t *)arg;
+    if (RCLASS_SERIAL(klass) > old_serial) {
+        return;
+    }
+
     mjit_remove_class_serial(RCLASS_SERIAL(klass));
     RCLASS_SERIAL(klass) = rb_next_class_serial();
 
@@ -99,7 +104,8 @@ rb_clear_method_cache_by_class(VALUE klass)
 	    INC_GLOBAL_METHOD_STATE();
 	}
 	else {
-	    rb_class_clear_method_cache(klass, Qnil);
+	    rb_serial_t old_serial = rb_next_class_serial();
+	    rb_class_clear_method_cache(klass, (VALUE)&old_serial);
 	}
     }