Review request for JDK-8062401: User accessors require boxing and do not support optimistic types

[Vladimir Ivanov]

> What are the performance implications? Have you done some octane runs or something?
>
> Furthermore, I’d like to see a comment in the benchmark you’ve added, what it’s for, a reference to the issue number JDK-8062401 in a comment and what the difference before and after was.
>
> Otherwise - solid work.  Nicely done.
>
> Have you discussed with Vladimir Ivanov, the problem he had with box2d and the inlining site in UserAccessorProperty? Will this affect it and make it better? CC:ing Vladimir.
I looked at box2d w/ the fix and inlining for accessors works fine. It 
improves Box2D peak performance by 10%.

Good work, Hannes! Thanks for fixing this!

Best regards,
Vladimir Ivanov