RFR: 8257641: Shenandoah: Query is_at_shenandoah_safepoint() from control thread should return false [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Update

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1600/files
  - new: https://git.openjdk.java.net/jdk/pull/1600/files/f87cc538..c394ceea

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1600&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1600&range=00-01

  Stats: 8 lines in 2 files changed: 2 ins; 3 del; 3 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