RFR: 8332094: GenShen: Reuse existing code to verify usage before rebuilding free set

[William Kemper]

We had some code in shHeap that largely duplicated existing functionality in the shVerify. This change just reuses the existing code (which has nicer assert messages).

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Commit messages:
 - Reuse existing generational usage check, cleanup some warning in verifier

Changes: https://git.openjdk.org/shenandoah/pull/432/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=432&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332094
  Stats: 104 lines in 5 files changed: 11 ins; 62 del; 31 mod
  Patch: https://git.openjdk.org/shenandoah/pull/432.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/432/head:pull/432

PR: https://git.openjdk.org/shenandoah/pull/432