[jdk17u-dev] RFR: 8272327: Shenandoah: Avoid enqueuing duplicate string candidates

[Aleksey Shipilev]

Clean backport to improve Shenandoah.

Additional testing:
 - [x] Linux x86_64 fastdebug `hotspot_gc_shenandoah`

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Commit messages:
 - Backport ee8bf10d321da8a261ff4eda705cef753b4a7014

Changes: https://git.openjdk.java.net/jdk17u-dev/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u-dev&pr=23&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272327
  Stats: 10 lines in 3 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk17u-dev/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u-dev pull/23/head:pull/23

PR: https://git.openjdk.java.net/jdk17u-dev/pull/23