RFR: 8344049: Shenandoah: Eliminate init-update-refs safepoint [v2]

[William Kemper]

> Not clean, has two follow up fixes in this PR.

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

 - Allow UPDATE_REFS in gc state when verifying before update refs
 - Update gc state resetter for verifier
 - Remove code block missed when IU mode was removed

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

Changes:
  - all: https://git.openjdk.org/shenandoah-jdk21u/pull/161/files
  - new: https://git.openjdk.org/shenandoah-jdk21u/pull/161/files/fcb58bb7..219c4ef2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=161&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=161&range=00-01

  Stats: 25 lines in 5 files changed: 14 ins; 8 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/161.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/161/head:pull/161

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/161