RFR 8210856: Move InstanceKlass DependencyContext cleaning to SystemDictionary::do_unloading()

[coleen.phillimore at oracle.com]

Summary: Already walk classes in ClassLoaderData::unload so generalize 
to also clean nmethod dependencies.

The nmethod dependencies should be cleaned before metadata is purged.  
See bug for more details.

open webrev at http://cr.openjdk.java.net/~coleenp/8210856.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8210856

Tested with hs tier1-7, modified gtest, and 
compiler/codecache/stress/RandomAllocationTest.java from bug 
https://bugs.openjdk.java.net/browse/JDK-8143408.

Thanks,
Coleen