This is a follow-up to JDK-8305895. Performance testing has revealed some regressions on performance even when compact headers are turned off. For example, the RSA benchmark of SPECjvm was ~2% slower when compared to the baseline JDK. Those regressions seem to be caused by oopDesc::klass() now checking two global variables rather than previously only one. This potentially also harms GC iterators, which do a lot of Klass* accesses.