RFR: 8340381: Shenandoah: Class mirrors verification should check forwarded objects

[Aleksey Shipilev]

The from-space objects can be effectively dead, and their backlinks to `InstanceKlass*` not updated anymore. So they can point to garbage. 

Additional testing:
 - [x] Some previously failing reproducers are not failing anymore
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21064/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340381
  Stats: 22 lines in 2 files changed: 9 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/21064.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21064/head:pull/21064

PR: https://git.openjdk.org/jdk/pull/21064