RFR: 8227407: ZGC: C2 loads and load barriers can get separated by safepoints

[Roland Westrelin]

> How would you feel about adding a new enum value for UnsafePinned, and 
> modifying the LoadNode::Ideal transformation to
> explicitly allow the floating if the !depends_only_on_test() || 
> is_unsafe_pinned()?

That's fine with me but the name UnsafePinned wouldn't work because
Pinned is currently used for one thing other than unsafe: some arraycopy
related optimizations. UnknownControlDependency or something shorter?

Roland.