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

[William Kemper]

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

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Fix zero build
 - Fix comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/410/files
  - new: https://git.openjdk.org/shenandoah/pull/410/files/2ed3d6d7..8ebe8b8c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=00-01

  Stats: 7 lines in 3 files changed: 4 ins; 3 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