RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v20]

[Kelvin Nilsen]

> The mainline implementation of ShenandoahFreeSet was recently updated.  This PR integrates the upstream changes
> into Generational Shenandoah.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Add instrumentation to debug rare regression

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/7a0c4817..ab4e2015

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=19
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=18-19

  Stats: 200 lines in 4 files changed: 200 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/440.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/440/head:pull/440

PR: https://git.openjdk.org/shenandoah/pull/440