During the work for 6829187 we have fixed a number of basic bugs which are logically grouped with 6815692 and 6858164 but which must be reviewed and pushed separately.
Reviewed-by: kvn, never
6889757: G1: enable card mark elision for initializing writes from compiled code (ReduceInitialCardMarks)
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
