RFR: 8310680: GenShen: In-place region promotions may fail [v3]

[William Kemper]

> There are a few small changes here:
> * In `op-update-refs-final`, only active regions are aged.
> * In `op-final-roots`, a region's age may be reset outside of an aging cycle.
> * When the field used to save `top` before promotion is set, it indicates the region has been chosen for in-place-promotion.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Assert that promotion conditions hold, rather than rechecking

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/290/files
  - new: https://git.openjdk.org/shenandoah/pull/290/files/7fc3374d..bbce1a88

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=290&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=290&range=01-02

  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/290.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/290/head:pull/290

PR: https://git.openjdk.org/shenandoah/pull/290