RFR: 8309956: Shenandoah: Strengthen the mark word check in string dedup

[William Kemper]

On Tue, 13 Jun 2023 19:10:15 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the bug for details.
> 
> Additional testing:
>  - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

LGTM.

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Marked as reviewed by wkemper (no project role).

PR Review: https://git.openjdk.org/jdk/pull/14455#pullrequestreview-1478078790