[turbofan] Properly ensure that deoptimization is enabled.

test/mjsunit/regress/regress-crbug-687990.js

Index: test/mjsunit/regress/regress-crbug-687990.js
diff --git a/test/mjsunit/regress/regress-crbug-687990.js b/test/mjsunit/regress/regress-crbug-687990.js
new file mode 100644
index 0000000000000000000000000000000000000000..24100854c4c47153d90fb02b414b127bc625ada4
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-687990.js
@@ -0,0 +1,16 @@
+// 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
+
+var x = 1;
+
+var foo = (function() {
+  "use asm";
+  var o = this;
+  return function() { o.x = null; }
+})();
+
+%OptimizeFunctionOnNextCall(foo);
+foo();