RFR (7xS): 8071280: Specialize HeapRegion::oops_on_card_seq_iterate_careful for use during concurrent refinement and updating the rset

[Kim Barrett]

> On May 5, 2017, at 7:30 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> New webrevs:
> http://cr.openjdk.java.net/~tschatzl/8071280/webrev.0_to_1/ (diff)
> http://cr.openjdk.java.net/~tschatzl/8071280/webrev.1/ (full)

Looks good.

Do the tails of block_size and block_size_during_gc remain similar through
all of these cleanups?  If so, merging them into a common helper would be
nice.  That can be a further cleanup if they do remain similar.