RFR: 8325673: GenShen: Share Reserves between Old and Young Collector [v8]

[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:

  Simplify invocations of freeset rebuild when possible
  
  Most invocations do not need to resize generations between
  prepare_to_rebuild() and finish_rebuild(), so no need to make these two
  independent invocations.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/395/files
  - new: https://git.openjdk.org/shenandoah/pull/395/files/02ea5660..10588379

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=07
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=06-07

  Stats: 37 lines in 6 files changed: 12 ins; 20 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