RFR: 8325673: GenShen: Share Reserves between Old and Young Collector [v7]

[Kelvin Nilsen]

> Allow young-gen Collector reserve to share memory with old-gen Collector reserve in order to support prompt processing of mixed evacuations, as constrained by ShenandoahOldEvacRatioPercent.

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

  Better comments as requested by code review

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/395/files
  - new: https://git.openjdk.org/shenandoah/pull/395/files/21a5d328..02ea5660

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=06
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=05-06

  Stats: 20 lines in 2 files changed: 14 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/395.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/395/head:pull/395

PR: https://git.openjdk.org/shenandoah/pull/395