RFR: Improve handling of cancellation after final mark [v2]

[Kelvin Nilsen]

On Fri, 4 Feb 2022 00:11:08 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Handle case when gc is cancelled _after_ final mark but _before_ the control thread checks for cancellation. If final mark completes before the cancellation is detected, we can no longer degenerate to the marking phase.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add clarifying comment

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/109