Do not switch to two-byte string in String.fromCharCode if avoidable.

test/mjsunit/regress/string-fromcharcode-sideeffect.js

+// Copyright 2015 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.
+
+var counter = 0;
+var a = { valueOf: function() { counter++; return 0x100; } };
+assertEquals("A\u0100\u0100\u0100", String.fromCharCode(65, a, a, a));
+assertEquals(3, counter);