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

[Kelvin Nilsen]

On Wed, 26 Mar 2025 19:09:04 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> 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

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah-jdk21u/pull/161#pullrequestreview-2723198393