Improve write barriers in optimized code.

Use a cheaper RecordWriteForMap() to update the
write barrier for maps. And skip the value check
in RecordWriteField() when we statically know that
the value is in new space (and therefore has "pointers
to here are interesting" flag set).

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/297763006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21630 ce2b1a6d-e550-0410-aec6-3dcde31c8c00