RFR: 8309956: Shenandoah: Strengthen the mark word check in string dedup [v2]

[Aleksey Shipilev]

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

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8309956-shenandoah-strdedup-check
 - Fix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14455/files
  - new: https://git.openjdk.org/jdk/pull/14455/files/facfb975..fb8a70b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14455&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14455&range=00-01

  Stats: 68073 lines in 1080 files changed: 50776 ins; 13698 del; 3599 mod
  Patch: https://git.openjdk.org/jdk/pull/14455.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14455/head:pull/14455

PR: https://git.openjdk.org/jdk/pull/14455