[turbofan] Properly handle IfException projections on JSForInNext.

test/mjsunit/regress/regress-6121.js

Index: test/mjsunit/regress/regress-6121.js
diff --git a/test/mjsunit/regress/regress-6121.js b/test/mjsunit/regress/regress-6121.js
new file mode 100644
index 0000000000000000000000000000000000000000..c29ba2b067e86234cf90933dd8e4b9f31a98b68f
--- /dev/null
+++ b/test/mjsunit/regress/regress-6121.js
@@ -0,0 +1,23 @@
+// 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
+
+function foo(o) {
+  try {
+    for (var x in o) {}
+    return false;
+  } catch (e) {
+    return true;
+  }
+}
+
+var o = new Proxy({a:1},{
+  getOwnPropertyDescriptor(target, property) { throw target; }
+});
+
+assertTrue(foo(o));
+assertTrue(foo(o));
+%OptimizeFunctionOnNextCall(foo);
+assertTrue(foo(o));