Revert "Faster rb_class_superclass"

This reverts commit 29b68b89a0.

object.c

@@ -2034,18 +2034,19 @@ rb_class_new_instance(int argc, const VALUE *argv, VALUE klass)
 VALUE
 rb_class_superclass(VALUE klass)
 {
-    RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));
-
     VALUE super = RCLASS_SUPER(klass);
 
     if (!super) {
 	if (klass == rb_cBasicObject) return Qnil;
 	rb_raise(rb_eTypeError, "uninitialized class");
-    } else {
-        super = RCLASS_SUPERCLASSES(klass)[RCLASS_SUPERCLASS_DEPTH(klass) - 1];
-        RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));
-        return super;
     }
+    while (RB_TYPE_P(super, T_ICLASS)) {
+	super = RCLASS_SUPER(super);
+    }
+    if (!super) {
+	return Qnil;
+    }
+    return super;
 }
 
 VALUE