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

[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.

-------------

Commit messages:
 - Only arm nmethod barriers if collection set is not empty

Changes: https://git.openjdk.org/shenandoah/pull/286/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=286&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309874
  Stats: 6 lines in 1 file changed: 2 ins; 1 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