RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v12]

[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:

  Make complex assertion a bit easier to read

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/d675bed4..1bc1c4fb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=11
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=10-11

  Stats: 36 lines in 2 files changed: 22 ins; 8 del; 6 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