RFR: 8257641: Shenandoah: Query is_at_shenandoah_safepoint() from control thread should return false

[Zhengyu Gu]

Since Shenandoah GC safepoints are scheduled by control thread, so that, if querying comes from control thread, the answer should be false.

is_at_shenandoah_safepoint() is still not reliable, even after JDK-8253778, we may consider to scratch it.

- [x] hotspot_gc_shenandoah x86_64 and x86_32

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Commit messages:
 - JDK-8257641

Changes: https://git.openjdk.java.net/jdk/pull/1600/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1600&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257641
  Stats: 11 lines in 2 files changed: 9 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1600.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1600/head:pull/1600

PR: https://git.openjdk.java.net/jdk/pull/1600