ARM64: do not optimize away shift if its result is used as both left and right

test/mjsunit/regress/regress-491049.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.
 
-// Flags: --allow-natives-syntax
-
-function f() { return f.x; }
-f.__proto__ = null;
-f.prototype = "";
+function f(a) {
+  return (a >>> 12) + (a >>> 2);
+}
 
 f();
+
+%OptimizeFunctionOnNextCall(f);
+
 f();
-%OptimizeFunctionOnNextCall(f);
-f();