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

[William Kemper]

`tail_mask` function should not be in the global scope.

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Commit messages:
 - Fix name collision for tail_mask function

Changes: https://git.openjdk.org/jdk/pull/21395/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21395&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/jdk/pull/21395.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21395/head:pull/21395

PR: https://git.openjdk.org/jdk/pull/21395