Shenandoah crashes when running compiler/c2/cr6865031 test

[Roland Westrelin]

> It appears that it crashes when pointing test jdk to
> /build/linux-x86_64-normal-server-fastdebug/jdk, but works fine when
> linux-x86_64-normal-server-fastdebug/images/jdk is used.

I can reproduce it now. The problem is that the signal handler doesn't
recognize a load of the brooks pointer from a null object as an implicit
null check when compressed oops are on (subtracting the base doesn't
happen because the load of a brooks pointer on a null object hits right
below the base):

http://cr.openjdk.java.net/~roland/shenandoah/compressedoops-nullcheck/webrev.00/

Roland.