RFR: 8267767: Redundant condition check in SafepointSynchronize::thread_not_running

[David Holmes]

As per the discussion in the JBS issue this adds a comment and some logging to the second condition check.

Testing: tiers 1-5 builds only (no tests as no functional changes)

Thanks,
David

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Commit messages:
 - 8267767: Redundant condition check in SafepointSynchronize::thread_not_running

Changes: https://git.openjdk.java.net/jdk/pull/6552/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6552&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267767
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6552.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6552/head:pull/6552

PR: https://git.openjdk.java.net/jdk/pull/6552