[turbofan] Properly allocate constant-folded string.

test/mjsunit/regress/regress-crbug-664942.js

Index: test/mjsunit/regress/regress-crbug-664942.js
diff --git a/test/mjsunit/regress/regress-crbug-664942.js b/test/mjsunit/regress/regress-crbug-664942.js
new file mode 100644
index 0000000000000000000000000000000000000000..3b769905118c79e6514f319bdafef5d6ff1690ed
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-664942.js
@@ -0,0 +1,12 @@
+// Copyright 2016 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: --allow-natives-syntax
+
+function foo() {
+  return 'x'[0];
+}
+foo();
+%OptimizeFunctionOnNextCall(foo);
+assertEquals("x", foo());