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

[William Kemper]

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

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

Commit messages:
 - Do not limit old generation available by mutator freeset available

Changes: https://git.openjdk.org/shenandoah/pull/301/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312610
  Stats: 26 lines in 3 files changed: 15 ins; 10 del; 1 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