RFR: Changes to simplify MU accounting

[Kelvin Nilsen]

On Thu, 11 May 2023 21:39:16 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Or may be I am misunderstanding what "repeat" means here.
>
> I think only a boot strap cycle and its first old marking increment will have the same `gc_id`.

I'm updating this comment on the expand-old-on-demand branch to try to make this more clear.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/274#discussion_r1202822263