RFR: 8325303: Replace markWord.is_neutral() with markWord.is_unlocked()

[David Holmes]

This is a small tidy up to try and remove confusion between checking `is_neutral` (a general state normally associated with a displaced markword in a "pristine" state) and `is_unlocked` (a specific state within the locking protocol). The underlying bit-pattern is the same and so these have been used somewhat synonymously/interchangeably.

A few comment tweaks too.

Testing: tiers 1-3 (sanity)

Thanks.

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Commit messages:
 - Merge
 - 8325303: Replace markWord.is_neutral() with markWord.is_unlocked()

Changes: https://git.openjdk.org/jdk/pull/17741/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325303
  Stats: 26 lines in 5 files changed: 2 ins; 2 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/17741.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17741/head:pull/17741

PR: https://git.openjdk.org/jdk/pull/17741