RFR: 8309874: NMethod barriers may remain armed when regions are promoted in place [v3]

[William Kemper]

> Genshen runs in-place promotions during the evacuation phase. If the cycle is _only_ performing in-place promotions, it should not arm NMethod barriers. The barriers are now only armed and disarmed if the collection set is not empty.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'shenandoah-master' into do-not-arm-nmethods-for-in-place-promotion
 - Describe nmethod barrier arming/disarming protocol in comments
 - Only arm nmethod barriers if collection set is not empty

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/286/files
  - new: https://git.openjdk.org/shenandoah/pull/286/files/715da332..e92741f9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=286&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=286&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/286.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/286/head:pull/286

PR: https://git.openjdk.org/shenandoah/pull/286