RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode [v3]

[William Kemper]

> Other modes should not instantiate or touch young/old generations.

William Kemper has updated the pull request incrementally with three additional commits since the last revision:

 - Make age and youth const
 - Do not age regions after completing old mark
 - Detect cancellation of old GC after final mark.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/444/files
  - new: https://git.openjdk.org/shenandoah/pull/444/files/b28513f7..d8215be6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=01-02

  Stats: 14 lines in 5 files changed: 8 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/444.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/444/head:pull/444

PR: https://git.openjdk.org/shenandoah/pull/444