RFR: 8253361: Shenandoah: runtime barrier does not honor ShenandoahSelfFixing flag

[Zhengyu Gu]

ShenandoahSelfFixing flag is currently only honored by C2, not runtime barrier. Runtime barrier should check the check
too.

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Commit messages:
 - 8253361: Shenandoah: runtime barrier does not honor ShenandoahSelfFixing flag

Changes: https://git.openjdk.java.net/jdk/pull/257/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=257&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253361
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/257.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/257/head:pull/257

PR: https://git.openjdk.java.net/jdk/pull/257