RFR: 8318057: GenShen: investigate small refactor of remset swap code

[Y. Srinivas Ramakrishna]

Some closures were being unnecessarily applied to free regions rather than just to allocated old and young regions. Restrict the closure application to old and young regions as appropriate, eliminating extra generation affiliation checks in some closures. Also fixed a slightly incorrect documentation comment.

**Testing:**
- jtreg and stress tests in progress in test pipeline helper
- locally tested with jtreg hotsot_gc tests and with specjbb with and without heap verification
- performance differences, if any, to be measured

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Commit messages:
 - Merge branch 'master' into swap_rs
 - fix for comp error (wrong variable name).
 -     Avoid including free young regions in young gen heap region
 - Avoid including free old regions in old gen heap region

Changes: https://git.openjdk.org/shenandoah/pull/342/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=342&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318057
  Stats: 16 lines in 2 files changed: 2 ins; 3 del; 11 mod
  Patch: https://git.openjdk.org/shenandoah/pull/342.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/342/head:pull/342

PR: https://git.openjdk.org/shenandoah/pull/342