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

[William Kemper]

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

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Commit messages:
 - 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/32/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=32&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328626
  Stats: 276 lines in 10 files changed: 101 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