RFR: 8253361: Shenandoah: runtime barrier does not honor ShenandoahSelfFixing flag [v2]

[Zhengyu Gu]

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

Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev
excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since
the last revision:

 - Merge branch 'JDK-8253361' of github.com:zhengyu123/jdk into JDK-8253361
 - Honor ShenandoahSelfFixing flag in runtime barrier
 - Tighten recusive reserveration handling
 - Fix trailing whitespace
 - Restore assertion
 - JDK-8252921
 - JDK-8252921
 - Honor ShenandoahSelfFixing flag in runtime barrier

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/254/files
  - new: https://git.openjdk.java.net/jdk/pull/254/files/739124ff..953e9173

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=254&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=254&range=00-01

  Stats: 7549 lines in 261 files changed: 3641 ins; 3125 del; 783 mod
  Patch: https://git.openjdk.java.net/jdk/pull/254.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/254/head:pull/254

PR: https://git.openjdk.java.net/jdk/pull/254