RFR (L, tedious) 8160399: is_oop_or_null involves undefined behavior

[Kim Barrett]

> On Aug 17, 2017, at 7:30 PM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi Coleen,
> 
> Could you motivate why you changed is_oop() to a static functions?

Quoting myself from a comment in 8160399:

For it [is_oop] to fail one has to take a non-oop, make it appear to be an oop, and 
call an oop member function on that not really an oop. That sounds like UB to me.