[deoptimizer] Materialize string iterators.

test/mjsunit/regress/regress-683617.js

+// Copyright 2017 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
+
+global = 'n';
+function f(deopt) {
+  let it = global[Symbol.iterator]();
+  if (deopt) {
+    return it.next().value;
+  }
+}
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+assertEquals('n', f(true));