JDK-8368875 saw that we had to preserve the markWord for ParallelGC. This issue represents an investigation into whether this is necessary for other GCs, and to take action if that is the case.

Original investigative prompt:
Preserving the markWord only doesn't work for Serial GC, if changing oopDesc::init_mark() to not look in the Klass header for the prototype_header, but does work for the other GCs, and fixes the linked bug JDK-8368875 for ParallelGC. Since in ParallelGC it's unsafe to look at the klass() pointer in MoveAndUpdateClosure::do_addr().