[jdk17u-dev] RFR: 8309956: Shenandoah: Strengthen the mark word check in string dedup

[Aleksey Shipilev]

Clean backport to improve Shenandoah reliability.

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

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Commit messages:
 - Backport 57b8251241e2044d5039ce162bf4637a9b2e5466

Changes: https://git.openjdk.org/jdk17u-dev/pull/1443/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1443&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/jdk17u-dev/pull/1443.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1443/head:pull/1443

PR: https://git.openjdk.org/jdk17u-dev/pull/1443