[parsing] Fix maybe-assigned for loop variables.

test/mjsunit/regress/regress-5636-1.js

+// 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-context-specialization
+
+function f(n) {
+  var a = [];
+  function g() { return x }
+  for (var i = 0; i < n; ++i) {
+    var x = i;
+    a[i] = g;
+    %OptimizeFunctionOnNextCall(g);
+    g();
+  }
+  return a;
+}
+var a = f(3);
+assertEquals(3, a.length);
+assertEquals(2, a[0]());
+assertEquals(2, a[1]());
+assertEquals(2, a[2]());