RFR: 8212992: Change mirror accessor in Klass::verify_on() to use AS_NO_KEEPALIVE

[Erik Österlund]

Hi,

The Klass::verify_on() is sometimes called on unloading klasses. It then 
reads a dead mirror, without using AS_NO_KEEPALIVE, which can make GC 
barriers feel like :C (because that should *never* normally happen). In 
order to make barriers happy, AS_NO_KEEPALIVE should be used when 
loading dead oops.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8212992/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8212992

Thanks,
/Erik