[regexp] Ensure there are no shape changes on the fast path

test/mjsunit/regress/regress-6210.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --predictable
+
+const str = '2016-01-02';
+
+function testToUint32InSplit() {
+  var re;
+  function toDictMode() {
+    re.x = 42;
+    delete re.x;
+    return "def";
+  }
+
+  re = /./g;  // Needs to be global to trigger lastIndex accesses.
+  return re[Symbol.replace]("abc", { valueOf: toDictMode });
+}
+
+function testToStringInReplace() {
+  var re;
+  function toDictMode() {
+    re.x = 42;
+    delete re.x;
+    return 42;
+  }
+
+  re = /./g;  // Needs to be global to trigger lastIndex accesses.
+  return re[Symbol.split]("abc", { valueOf: toDictMode });
+}
+
+testToUint32InSplit();
+testToStringInReplace();