Fix very subtle bug in HNode.changeUse.

tests/language/partial_min_test.dart

+// 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.
+
+// This test triggered an NPE in dart2js.
+// The bug needed:
+// - double usage of the same variable in one expression ("b != b").
+// - replacement of "b" with "typechecked b".
+// - ...
+// The example below is minimal (as far as I was able to do).
+
+num foo(num a, num b) {
+  if (a > b) return b;
+  if (b is double) {
+    if (true) {
+      if (true) {
+        return (a + b) * a * b;
+      }
+    }
+    // Check for NaN and b == -0.0.
+    if (a == 0 && b == 0 || b != b) return b;
+  }
+}
+
+main() {
+  Expect.equals(1, foo(2, 1));
+}