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

[Aleksey Shipilev]

Clean backport to improve Shenandoah reliability.

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Commit messages:
 - Backport 57b8251241e2044d5039ce162bf4637a9b2e5466

Changes: https://git.openjdk.org/jdk21/pull/19/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=19&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/jdk21/pull/19.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/19/head:pull/19

PR: https://git.openjdk.org/jdk21/pull/19