VM: Fix bug in type propagation at conditionals.

tests/language/vm/type_propagation_test.dart

+// Copyright (c) 2015, 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.
+// VMOptions=--optimization-counter-threshold=1000 --max-polymorphic-checks=1
+
+// Test correct loop invariant code motion and type propagation from is-checks
+// and null-comparisons. 
+
+class B {
+  var b;
+  B(this.b);
+}
+
+class C {
+  final f0;
+
+  final a;
+  C() : a = new B(0);
+}
+
+foo(x) {
+  for (var i = 0; i < 10; i++) {
+    i + i;
+    i + i;
+    if (x is C) {
+      x.a.b < 0;
+    }
+  }
+}
+
+class Y { var f = null; }
+
+bar(y) {
+  var x = y.f;
+  for (var i = 0; i < 10; i++) {
+    if (x != null) {
+      x.a.b < 0;
+    }
+  }
+}
+
+
+main () {
+  var o = new Y();
+  o.f = new C();
+  bar(o);
+  o.f = null;
+  bar(o);
+
+  for (var i = 0; i < 1000; i++) bar(o);
+
+  foo(new C());
+  foo(0);
+
+  for (var i = 0; i < 1000; i++) foo(0);
+}