RFR: 8309874: NMethod barriers may remain armed when regions are promoted in place [v2]

[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 incrementally with one additional commit since the last revision:

  Describe nmethod barrier arming/disarming protocol in comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/286/files
  - new: https://git.openjdk.org/shenandoah/pull/286/files/fac642a3..715da332

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=286&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=286&range=00-01

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 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