RFR: 8310680: Improvements for region aging and in-place-promotions

[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.

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Commit messages:
 - Improvements for region aging and in-place-promotions

Changes: https://git.openjdk.org/shenandoah/pull/290/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=290&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310680
  Stats: 42 lines in 5 files changed: 25 ins; 7 del; 10 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