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

Reviewed-by: thartmann, eosterlund, kvn

Author: Tom Rodriguez

src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/HotSpotResolvedObjectTypeImpl.java

@@ -265,7 +265,12 @@ private AssumptionResult<ResolvedJavaType> concreteSubtype(HotSpotResolvedObject
      * @return true if the type is a leaf class
      */
     private boolean isLeafClass() {
-        return compilerToVM().getResolvedJavaType(this, config().subklassOffset, false) == null;
+        // In general, compilerToVM().getResolvedJavaType should always be used to read a Klass*
+        // from HotSpot data structures but that has the side effect of creating a strong reference
+        // to the Class which we do not want since it can cause class unloading problems.  Since
+        // this code is only checking for null vs non-null so it should be safe to perform this
+        // check directly.
+        return UNSAFE.getLong(this.getKlassPointer() + config().subklassOffset) == 0;
     }
 
     /**