[regexp] Avoid side effects between map load and fast path check

test/mjsunit/regress/regress-709029.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.
+
+// Changes the map of obj.
+function mutateObjectOnStringConversion(obj) {
+  return { toString: () => { obj.x = 42;return "";}};
+}
+
+{
+  const re = /./;
+  re.exec(mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re.test(mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.match](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.search](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.split](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.replace](mutateObjectOnStringConversion(re));
+}