RFR: 8364111: InstanceMirrorKlass iterators should handle CDS and hidden classes consistently

[Aleksey Shipilev]

See the bug for more investigation. I think the root cause is already fixed in one of the `InstanceMirrorKlass` iterator, but not in the other one. Additionally, only one iterator handles the hidden classes. Also, there is a stale comment about CDS that apparently prevents us from asserting `is_primitive`, which is not relevant anymore.

So this PR commons out the metadata handling on both iterators, which fixes the GenShen+CDS bug, likely fixes hidden classes bug somewhere, and prevents the omission like this to happen again.

Additional testing:
 - [x] Linux x86_64 server fastdebug, Generational Shenandoah + CDS bugs are not reproducing now
 - [x] Linux x86_64 server fastdebug, `tier1`
 - [ ] Linux x86_64 server fastdebug, `all`

-------------

Commit messages:
 - Comments
 - Mark as inline as well
 - Fix

Changes: https://git.openjdk.org/jdk/pull/26477/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26477&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364111
  Stats: 59 lines in 2 files changed: 27 ins; 30 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26477.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26477/head:pull/26477

PR: https://git.openjdk.org/jdk/pull/26477