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

[Aleksey Shipilev]

See the bug for details.

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/14455/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14455&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309956
  Stats: 8 lines in 1 file changed: 1 ins; 0 del; 7 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