[turbofan] Fix deopt check for storing into constant field.

test/mjsunit/compiler/regress-626986.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 g() {
+  return 42;
+}
+
+var o = {};
+
+function f(o, x) {
+  o.f = x;
+}
+
+f(o, g);
+f(o, g);
+f(o, g);
+assertEquals(42, o.f());
+%OptimizeFunctionOnNextCall(f);
+f(o, function() { return 0; });
+assertEquals(0, o.f());