The current way classes are pre-loaded during a class loading phase suffers from two issues:
  - it cannot be extended to support speculative loadings (triggered by JEP 401 Preload attribute)
  - it doesn't support concurrent class loadings when pre-loading classes while checking for circularity issue

The existing loading code used to load super-types, which requires check for circularity too, could be slightly modified to become less super-types specific and usable for both super-types and speculative loadings.