RFR: Recompute generation accounts after fullgc [v2]

[Kelvin Nilsen]

> This fix addresses a problem found during verification following full gcs.  This recomputes the generation usage at the end of full GC as part of a 5th phase of full gc.
> 
> We may be able to remove this 5th phase at a later time after we find and fix whatever errors exist in generation accounting in the existing full gc implementation.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Reviewer feedback: change function name to ...restore_generation_usage()

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/235/files
  - new: https://git.openjdk.org/shenandoah/pull/235/files/b4c9c2d6..0e58a72e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=235&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=235&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/235.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/235/head:pull/235

PR: https://git.openjdk.org/shenandoah/pull/235