RFR: 8321225: [JVMCI] HotSpotResolvedObjectTypeImpl.isLeafClass shouldn't create strong references

[Tom Rodriguez]

Checking for leaf Klasses requires seeing if the subklass field is null.  As part of the fix for JVMCI support for ZGC, JDK-8299229, it was changed to call into the runtime which had the side effect of creating a strong reference to an the class.  Since it's only checking for non-null it's ok to just perform thread directly as was done prior to JDK-8299229.  This avoids causing class unloading problems.

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Commit messages:
 - 8321225: [JVMCI] HotSpotResolvedObjectTypeImpl.isLeafClass shouldn't create strong references

Changes: https://git.openjdk.org/jdk/pull/16943/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321225
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16943/head:pull/16943

PR: https://git.openjdk.org/jdk/pull/16943