[jdk11u-dev] RFR: 8276201: Shenandoah: Race results degenerated GC to enter wrong entry point

[Zhengyu Gu]

This is a Shenandoah specific backport. It fixes a race that results degenerated GC to enter wrong degen entry point.

The original patch does not apply cleanly due to code divergence, especially the grand refactoring that is not in jdk11u. However, the backport is simple and straight forward.

Test:
- [x] hotspot_gc_shenandoah

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

Commit messages:
 - Backport dbf5100dd705fbe4a3aeae49405ca541d581f106

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/612/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=612&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276201
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/612.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/612/head:pull/612

PR: https://git.openjdk.java.net/jdk11u-dev/pull/612