Description:
The "8038212: Method::is_valid_method() check has performance regression
  impact for stackwalking" - changeset introduced a change in how the ClassLoaderDataGraph::_unloading list of ClassLoaderData's is purged.

This change to the purging of the CLD’s work the same as before for most GC's, but when using CMS GC, SystemDictionary::do_unloading() is called twice with no explicit purge call in between. On the second call (post-sweep), we can now get stale class loader oops delivered as part of the Klass closure callbacks from the _unloading list. Again, this is because there is no explicit purge call in between these two entries to SystemDictionary::do_unloading() - and being CMS and concurrent, it is very hard to accommodate a timely and proper purge call here.

The first do_unloading call comes after CMS concurrent marking, and the second comes from a Full GC triggered while sweeping the CMS heap.

This fix ensures the unloading purge mechanism to work correctly also for the CMS collector, in that only CLDs with non-reclaimed class loader oops will deliver klasses from the _unloading list. In addition, this will ensure a single "logical" pass is achieved when iterating the unloading list in-between purges (avoiding the processing of the same data twice).

This fix is precipitated by nightly testing failures with CMS after the introduction of 8038212: Method::is_valid_method() check has performance regression
  impact for stackwalking" - for example "nsk/sysdict/vm/stress/jck12a//sysdictj12a008" which is crashing because of following up stale klass loader oop's from the ClassLoaderDataGraph::_unloading list.

Additional nightly test failures tracked in: https://bugs.openjdk.java.net/browse/JDK-8044677