RFR: Disable redundant generation usage after fullgc

Tue Apr 4 15:23:55 UTC 2023

After a recent integration, we ended up recomputing generation usage twice.  This change integrates the two efforts with a single more efficient pass.

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Commit messages:
 - Merge remote-tracking branch 'GitFarmBranch/disable-redundant-generation-usage-after-fullgc' into disable-redundant-generation-usage-after-fullgc
 - Remove over-zealous asserts
 - Fixup single computation of generation usage
 - Eliminate redundant generational usage calculations after full gc

Changes: https://git.openjdk.org/shenandoah/pull/242/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=242&range=00
  Stats: 125 lines in 3 files changed: 46 ins; 69 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah/pull/242.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/242/head:pull/242

PR: https://git.openjdk.org/shenandoah/pull/242