RFR: 8312610: GenShen: Old generation available is unintentionally restricted by mutator's available memory [v3]

[William Kemper]

> Only young gen allocations should be restricted to memory in the mutator's view of the freeset.

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

 - Fix bug in `shYoungGen::soft_available`
 - Fix typo in comment
 - Assert that max capacity >= soft capacity

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/301/files
  - new: https://git.openjdk.org/shenandoah/pull/301/files/0df69803..d88574b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=301&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=301&range=01-02

  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/301.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/301/head:pull/301

PR: https://git.openjdk.org/shenandoah/pull/301