RFR: 8340364: JFR: Fix up Class mirror after event retransformation

[Aleksey Shipilev]

See the bug description for details. I am trying to come up with a small regression test for it, please look at the fix meanwhile.

Additional testing:
 - [x] Original Shenandoah verifier reproducer no longer crashes
 - [x] Linux x86_64 server fastdebug, `jdk_jfr`
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21054/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21054&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340364
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21054.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21054/head:pull/21054

PR: https://git.openjdk.org/jdk/pull/21054