Only emit typeguards if we think they are valuable.

frog/tests/leg/src/GVNTest.dart

 // Copyright (c) 2011, 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.
 
 #import("compiler_helper.dart");
 
 final String TEST_ONE = @"""
 void foo(bar) {
-  print(1 + bar);
-  print(1 + bar);
+  for (int i = 0; i < 1; i++) {
+    print(1 + bar);
+    print(1 + bar);
+  }
 }
 """;
 
 main() {
   String generated = compile(TEST_ONE, 'foo');
   RegExp regexp = const RegExp(@"1 \+ bar");
   Iterator matches = regexp.allMatches(generated).iterator();
   Expect.isTrue(matches.hasNext());
   matches.next();
   Expect.isFalse(matches.hasNext());
 }