Optimize num::~/, num::>> and num::<< for some cases.

tests/language/truncdiv_uint32_test.dart

Index: tests/language/truncdiv_uint32_test.dart
===================================================================
--- tests/language/truncdiv_uint32_test.dart	(revision 0)
+++ tests/language/truncdiv_uint32_test.dart	(revision 0)
@@ -0,0 +1,9 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+var a = [0xFFFFFFFF];
+
+main() {
+  if (a[0] ~/ 1 != 0xFFFFFFFF) throw 'Test failed';
+}