RFR: 8317007: Add bulk removal of dead nmethods during class unloading

[Thomas Schatzl]

Hi all,

  please review this change that bulk-removes dead nmethods for the STW collectors instead of unregistering nmethod by nmethod. This significantly speeds up the class unloading phase.
    
For G1, this is almost 100% the code that has been removed from the review for JDK-8315503.
    
For Serial and Parallel GC, the code is new.
    
This change does not try to improve the situation for concurrent collectors - this would at first glance require extending the scope of the `CodeCache_lock` which I did not want to do. See the CR for more details.
    
Also, no parallelism for Parallel GC: the existing data structure for code root remembered set is a linked list. There is in almost all cases no point in trying to parallelize what is basically a traversal of the linked list (with each element not having a lot of work to do). I filed JDK-8320067. There should already be a significant speedup with this change, as each `unregister_nmethod` call previously traversed the entire linked list to find that element (i.e. complexity reduction of O(n^2) -> O(n)).
    
One more comment:

*  So it would be possible to merge the `ScavengableNMethod::prune_nmethods_not_into_young` which removes elements from the code root sets that do no longer need to be remembered (due to object movement) with removing dead/unlinked nmethods. However, this would mean to put that class unloading code to the end of gc as during phase 1 the new locations which are relevant for pruning uninteresting code root remembered set entries did not move yet. I wanted to keep determining the unlinking nmethods and class/code unloading them together in the code, but I could be convinced to move it.
    
Testing: tier1-4

Depends on #16759, please review first.

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/16759

Commit messages:
 - 8317007

Changes: https://git.openjdk.org/jdk/pull/16767/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16767&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317007
  Stats: 123 lines in 20 files changed: 108 ins; 4 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/16767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16767/head:pull/16767

PR: https://git.openjdk.org/jdk/pull/16767