Fix a crash in the compiler where an ssa instruction was using itself.

tests/language/bailout2_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.
+
+var a;
+
+main() {
+  // Write a loop to force a bailout method on [A.foo].
+  for (int i = 0; i < 10; i++) {
+    if (a != null) new A().foo([]);
+    Expect.equals(42, new A().foo(new A()));
+  }
+}
+
+class A {
+  // In dart2js, the optimized version of foo tries to optimize the
+  // uses of a.length (which is used two times here: for the index,
+  // and for the bounds check), and that optmization used to crash
+  // the compiler.
+  foo(a) => a[a.length];
+
+  int get length => 42;
+  operator[] (index) => 42;
+}