RFR: 8368307: Shenandoah: get_next_bit_impl should special case weak and strong mark bits

[Kelvin Nilsen]

Check for weak bit or strong bit when giving special handling to the next bit found in first position of word.

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Commit messages:
 - check for weak bit or strong bit in first bit position

Changes: https://git.openjdk.org/jdk/pull/27476/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27476&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368307
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27476.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27476/head:pull/27476

PR: https://git.openjdk.org/jdk/pull/27476