MIPS: Improve write barriers in optimized code.

Port r21630 (a21ff10)

Original commit message:
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).

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/315653002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21657 ce2b1a6d-e550-0410-aec6-3dcde31c8c00