RFR: 8279993: Assert that a shared class is not loaded more than once

[Calvin Cheung]

Please review this small fix for adding a debug only counter `InstanceKlass::_shared_class_load_count` to ensure a shared class is loaded only once.

Passed tiers 1,2,3 testing.

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Commit messages:
 - 8279993: Assert that a shared class is not loaded more than once

Changes: https://git.openjdk.org/jdk/pull/14023/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14023&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8279993
  Stats: 4 lines in 2 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14023.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14023/head:pull/14023

PR: https://git.openjdk.org/jdk/pull/14023