RFR: 8205050: ZGC: Incorrect use of RootAccess in ZHeapIterator

[Per Liden]

In ZHeapIteratorRootOopClosure::do_oop() we're using 
RootAccess<>::oop_load() to load oops. However, as the comment right 
above that line suggests, that's incorrect and we should have a load 
barrier here. In fact, we used to have a load barrier here, but this was 
for some mysterious reason changed. We should revert back to the code 
that used to be there.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205050
Webrev: http://cr.openjdk.java.net/~pliden/8205050/webrev.0

cheers,
Per