6892749: assert(UseParNewGC || UseSerialGC || UseConcMarkSweepGC, "...") fails
Summary: Removed the assert: UseSerialGC is not necessarily always set when serial gc is being used.
Reviewed-by: jcoomes, jmasa, tonyp
6888898: CMS: ReduceInitialCardMarks unsafe in the presence of cms precleaning
6889757: G1: enable card mark elision for initializing writes from compiled code (ReduceInitialCardMarks)
Summary: Defer the (compiler-elided) card-mark upon a slow-path allocation until after the store and before the next subsequent safepoint; G1 now answers yes to can_elide_tlab_write_barriers().
Reviewed-by: jcoomes, kvn, never