Fix PrepareKeyedOperand on arm.

test/mjsunit/regress/regress-358057.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
+__v_0 = new Uint8ClampedArray(10);
+for (var i = 0; i < 10; i++) {
+  __v_0[i] = 0xAA;
+}
+function __f_12(__v_6) {
+  if (__v_6 < 0) {
+    __v_1 = __v_0[__v_6 + 10];
+    return __v_1;
+  }
+}
+
+assertEquals(0xAA, __f_12(-1));
+%OptimizeFunctionOnNextCall(__f_12);
+assertEquals(0xAA, __f_12(-1));