RFR: 8337213: Shenandoah: Add verification for class mirrors

[William Kemper]

Clean backport, improves verification.

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Commit messages:
 - Backport 156f0b4332bf076165898417cf6678d2fc32df5c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/120/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=120&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337213
  Stats: 34 lines in 2 files changed: 34 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/120.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/120/head:pull/120

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/120