Fix GVN for loops.

tests/language/gvn_test.dart

Index: tests/language/gvn_test.dart
===================================================================
--- tests/language/gvn_test.dart	(revision 0)
+++ tests/language/gvn_test.dart	(revision 0)
@@ -0,0 +1,21 @@
+// Copyright (c) 2012, 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.
+
+class A {
+  int x = 0;
+
+  foo(i) {
+    var start = x;
+    do {
+      x++;
+      i++;
+    } while (i != 10);
+  }
+}
+
+main() {
+  var a = new A();
+  a.foo(0);
+  Expect.equals(10, a.x);
+}