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

[Roland Westrelin]

Hi Erik,

> http://cr.openjdk.java.net/~eosterlund/8227407/webrev.02/

Pinned was initially for unsafe accesses where a load can implicitly
depend on some condition to be valid and shouldn't float above that
condition. In that use case stepping over a safepoint is ok. So I wonder
if it would make sense to extend ControlDependency with a new element
for your use case which is somewhat different instead of overloading the
meaning of Pinned.

Roland.