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

test/mjsunit/regress/regress-708247.js

Index: test/mjsunit/regress/regress-708247.js
diff --git a/test/mjsunit/regress/regress-708247.js b/test/mjsunit/regress/regress-708247.js
new file mode 100644
index 0000000000000000000000000000000000000000..7512791cd95e76ca909e658570fc41c282486d12
--- /dev/null
+++ b/test/mjsunit/regress/regress-708247.js
@@ -0,0 +1,26 @@
+// 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 t() {
+  var re;
+  function toDictMode() {
+    for (var i = 0; i < 100; i++) {  // Loop is required.
+      re.x = 42;
+      delete re.x;
+    }
+    return 0;
+  }
+
+  re = /-/g;  // Needs to be global to trigger lastIndex accesses.
+  re.lastIndex = { valueOf : toDictMode };
+  return re.exec(str);
+}
+
+for (var q = 0; q < 10000; q++) {
+  t();  // Needs repetitions to trigger a crash.
+}