RFR: 8312120: GenShen: Update OLD Live at end of Previous Marking after Full GC

[William Kemper]

Record total live memory for old generation following Full GC marking phase.

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Commit messages:
 - Fix merge error
 - Update live bytes in old generation after full gc STW mark
 - Fix assertions and update comments
 - Prepare for mixed collections after global cycle

Changes: https://git.openjdk.org/shenandoah/pull/306/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=306&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312120
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/306.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/306/head:pull/306

PR: https://git.openjdk.org/shenandoah/pull/306