RFR: 8341668: Shenandoah: assert(tail_bits < (idx_t)BitsPerWord) failed: precondition

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 45a6359588996d25e5e8dadebdcd8d6a00ef786f

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/122/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=122&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341668
  Stats: 3 lines in 2 files changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/122.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/122/head:pull/122

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/122