Fix assertion failure in JSBuiltinReducer::ReduceArrayIterator.

test/mjsunit/regress/regress-crbug-671576.js

Index: test/mjsunit/regress/regress-crbug-671576.js
diff --git a/test/mjsunit/regress/regress-crbug-671576.js b/test/mjsunit/regress/regress-crbug-671576.js
new file mode 100644
index 0000000000000000000000000000000000000000..b9c7bccedea76b1b16ab2a5c9c0cbb09709acf76
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-671576.js
@@ -0,0 +1,13 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function f() {
+  for (var i of [NaN].keys());
+}
+
+f();
+%OptimizeFunctionOnNextCall(f);
+f();