MIPS: Omit redundant smi checks in write barriers.

Port r19323 (929c047)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/160713002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19343 ce2b1a6d-e550-0410-aec6-3dcde31c8c00