ReduceInitialCardMarks elides card marks for TLAB allocations
in young gen (fine for CMS), but for the case of slow-path allocation
makes up by card-marking the allocated object (potentially in the
old gen) before initialization. This latter is unsafe wrt CMS' precleaning
which might scan the marked cards before they have been initialized
by the mutator and thus lose those updates.
Deleted incorrect comment/update here.