RFR: 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads

[Aleksey Shipilev]

There are lots of Shenandoah-related test failures in current runs, triggered by new assert in JDK-8212107. The assert
looks legitimate, and Shenandoah code should instead be more relaxed here, by checking it runs in VMThread before
accessing its state.

Testing:
  - [x] hotspot_gc_shenandoah

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Commit messages:
 - Comments
 - 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads

Changes: https://git.openjdk.java.net/jdk/pull/424/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=424&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253778
  Stats: 9 lines in 2 files changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/424.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/424/head:pull/424

PR: https://git.openjdk.java.net/jdk/pull/424