RFR (S): 8191567: Refactor ciInstanceKlass G1 keep alive barrier to use Access API.

[Erik Österlund]

Hi,

When creating a ciInstanceKlass handle, G1 might need a SATB barrier to 
keep "peeked" weak klass pointers alive during marking.
This should now be done with the Access API instead of manual calls to 
the G1 SATB barrier.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8191567

Webrev:
http://cr.openjdk.java.net/~eosterlund/8191567/webrev.00/

Thanks,
/Erik