RFR: 8332913: GenShen: Restore shared update refs iterator [v2]

[William Kemper]

On Fri, 24 May 2024 20:00:44 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.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restore card stats enable check

GHA failures are from a slip of the mouse when I was restoring the original code.

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PR Comment: https://git.openjdk.org/shenandoah/pull/439#issuecomment-2130340312