RFR: 8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

[Y. Srinivas Ramakrishna]

8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

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Commit messages:
 - Backport d2102347ea9c1199221ec33f4e721aefa1193cea

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/68/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=68&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328235
  Stats: 169 lines in 16 files changed: 135 ins; 1 del; 33 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/68.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/68/head:pull/68

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/68