RFR: 8325673: GenShen: Share Reserves between Old and Young Collector [v12]

[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 with a new target base due to a merge or a rebase. The pull request now contains 82 commits:

 - Merge remote-tracking branch 'origin/master' into share-collector-reserves
 - Merge branch 'openjdk:master' into master
 - Cleanups requested by code review
 - Simplify code to rebuild free set after abbreviated and old GC
 - Remove incorrect and unnecessary comments
 - 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.
 - Better comments as requested by code review
 - Improve comment
 - Remove unreferenced variables
 - Simplify arguments by using instance variables in ShenandoahOldHeuristics
 - ... and 72 more: https://git.openjdk.org/shenandoah/compare/44f5995b...0e555e81

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

Changes: https://git.openjdk.org/shenandoah/pull/395/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=11
  Stats: 1254 lines in 24 files changed: 658 ins; 350 del; 246 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