RFR: 8332913: GenShen: Restore shared update refs iterator

[Kelvin Nilsen]

On Fri, 24 May 2024 17:55:24 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Turns out, this iterator is shared between concurrent and degenerated GC cycles. It should not have been made into a local variable.

Good teamwork figuring this out...  Thanks.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/439#pullrequestreview-2077694536