Fix polymorphic keyed loads for SLOPPY_ARGUMENTS_ELEMENTS

test/mjsunit/regress/regress-crbug-350867.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.
+
+function f1(a, i) {
+  return a[i];
+}
+function f2(a, b, c, index) {
+  return f1(arguments, index);
+}
+
+f2(2, 3, 4, "foo");
+f2(2, 3, 4, "foo");
+assertEquals(11, f1([11, 22, 33], 0));
+assertEquals(22, f2(22, 33, 44, 0));