RFR: 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field [v2]

[William Kemper]

> Backport of 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge remote-tracking branch 'shenandoah-21/master' into backport-earthling-amzn-573618e6
 - 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field
   
   Reviewed-by: kdnilsen, ysr

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Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/32/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=32&range=01
  Stats: 274 lines in 9 files changed: 99 ins; 117 del; 58 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/32/head:pull/32

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/32