RFR: 8370527: Memory leak after 8316694: Implement relocation of nmethod within CodeCache

[Chad Rakoczy]

[JDK-8370527](https://bugs.openjdk.org/browse/JDK-8370527)

[JDK-8316694](https://bugs.openjdk.org/browse/JDK-8316694) introduced an `immutable_data_references_counter` which keeps track of the number of nmethods using the immutable data so it can be shared between relocated nmethods. The old code reads the counter, decrements the counter, and then checks the first read to see if it is zero. Since the check is performed on the initial read it will never be zero which causes immutable data to never be freed.

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Commit messages:
 - Update reference_count check to equals 1

Changes: https://git.openjdk.org/jdk/pull/28008/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28008&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370527
  Stats: 8 lines in 1 file changed: 6 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28008.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28008/head:pull/28008

PR: https://git.openjdk.org/jdk/pull/28008