Improve Array.shift() performance for small arrays.

test/mjsunit/array-shift2.js

+// Copyright 2014 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
+
+Object.defineProperty(Array.prototype, "1", {
+  get: function() { return "element 1"; },
+  set: function(value) { }
+});
+function test(array) {
+  array.shift();
+  return array;
+}
+assertEquals(["element 1",2], test(["0",,2]));
+assertEquals(["element 1",{}], test([{},,{}]));
+%OptimizeFunctionOnNextCall(test);
+assertEquals(["element 1",0], test([{},,0]));