RFR: 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads [v2]

[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

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Revert the not very valid change

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/424/files
  - new: https://git.openjdk.java.net/jdk/pull/424/files/054c5840..fe406017

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=424&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=424&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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