[jdk17u] RFR: 8276201: Shenandoah: Race results degenerated GC to enter wrong entry point

[Zhengyu Gu]

A clean backport of Shenandoah specific bug. The patch fixes a subtle race condition that results degenerated GC to enter wrong degen entry.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Backport dbf5100dd705fbe4a3aeae49405ca541d581f106

Changes: https://git.openjdk.java.net/jdk17u/pull/232/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=232&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276201
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/232.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/232/head:pull/232

PR: https://git.openjdk.java.net/jdk17u/pull/232