RFR: 8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

[Kelvin Nilsen]

On Sat, 13 Jul 2024 00:59:12 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> Clean backport of commit [d2102347](https://github.com/openjdk/shenandoah/commit/d2102347ea9c1199221ec33f4e721aefa1193cea) from the [openjdk/shenandoah](https://git.openjdk.org/shenandoah) repository.
> 
> The commit being backported was authored by Y. Srinivas Ramakrishna on 2 Jul 2024 and was reviewed by William Kemper.
> 
> **Testing:** *in progress, will be updated when completed*
>  - [ ] Code pipeline testing
>  - [x] jtreg local
>  - [x] [GHA](https://github.com/openjdk-bots/shenandoah-jdk21u/actions/runs/9915888290)

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah-jdk21u/pull/68#pullrequestreview-2176549175