RFR: 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field [v4]

[William Kemper]

> This is a follow up to https://github.com/openjdk/shenandoah/pull/406.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Update comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/410/files
  - new: https://git.openjdk.org/shenandoah/pull/410/files/d75de101..e25db762

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=02-03

  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/410.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/410/head:pull/410

PR: https://git.openjdk.org/shenandoah/pull/410