RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode

[William Kemper]

Other modes should not instantiate or touch young/old generations.

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Commit messages:
 - Add missing precompiled header
 - Fix merge error
 - Better assert messages for inappropriate generation access
 - Merge branch 'shenandoah/master' into move-young-old-generations
 - Back out generation getter/helpers (members will stay in sh heap)
 - Factor region age management into gen heap, remove duplication
 - Move generation size and generation initialization out of shHeap
 - Fix copy/pasta in ShenandoahGenerationalMemoryPool
 - Simplify generational memory pools
 - Merge branch 'syncup-worker-initialization' into move-young-old-generations
 - ... and 20 more: https://git.openjdk.org/shenandoah/compare/a32f36bb...5e744806

Changes: https://git.openjdk.org/shenandoah/pull/444/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333750
  Stats: 881 lines in 20 files changed: 469 ins; 318 del; 94 mod
  Patch: https://git.openjdk.org/shenandoah/pull/444.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/444/head:pull/444

PR: https://git.openjdk.org/shenandoah/pull/444