RFR: 8339616: GenShen: Introduce new state to distinguish promote-in-place phase as distinct from concurrent evacuation

[William Kemper]

Not clean, but not hard to resolve conflicts.

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Commit messages:
 - Backport fdd5ed9906e1437e9ba4c0c6eeb4af3dd74b3adc

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/105/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339616
  Stats: 217 lines in 14 files changed: 131 ins; 37 del; 49 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/105.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/105/head:pull/105

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/105