Fix a bug in the handling of Phi nodes.

tests/compiler/dart2js_extra/21351_test.dart

+// Copyright (c) 2014, 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 {
+  bool _flag = false;
+  bool get flag => _flag;
+}
+
+main () {
+  var value1, value2; 
+  var count = 0;
+
+  for (var x = 0; x < 10; x++) {
+    var otherThing = new A();
+    for (var dummy = 0; dummy < x; dummy++) {
+      otherThing._flag = !otherThing._flag;
+    }
+
+    value1 = value2;
+    value2 = otherThing;
+
+    if (value1 == null) continue;
+
+    if (value1.flag) count++;
+  }
+
+  if (count == 0) throw "FAIL";
+}
+