RFR: 8225227: ZGC: Be exact in what load barrier to use in ZHeapInspector

[Per Liden]

After JDK-8224675 (which has not been pushed yet), the ZHeapItarator can 
be more exact in what load barrier is used on roots, and the load 
barrier can be skipped all together on strong non-concurrent roots.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225227
Webrev: http://cr.openjdk.java.net/~pliden/8225227/webrev.0

/Per