[jdk22u] RFR: 8322503: Shenandoah: Clarify gc state usage

[William Kemper]

Clean backport. This is a follow up to https://bugs.openjdk.org/browse/JDK-8321815, which renames methods and changes comments and adds an assert for improved readability.

-------------

Commit messages:
 - Backport 51238c4bdbce48f6b719f7dcfe5b7df8b8c6d85b

Changes: https://git.openjdk.org/jdk22u/pull/101/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk22u&pr=101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322503
  Stats: 26 lines in 5 files changed: 8 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jdk22u/pull/101.diff
  Fetch: git fetch https://git.openjdk.org/jdk22u.git pull/101/head:pull/101

PR: https://git.openjdk.org/jdk22u/pull/101