Naive algorithm for to update RSets for a span is rather inefficient
as it performs many unnecessary operations (retrieving a mask, updating
it with the same bit as many pointers go into a single region).

Review URL: http://codereview.chromium.org/2727009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4849 ce2b1a6d-e550-0410-aec6-3dcde31c8c00