MIPS: Always use the StoreFieldStub to do the actual storing.

Port r22931 (2af0a29f)

BUG=
R=gergely.kis@imgtec.com

Review URL: https://codereview.chromium.org/440423004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22940 ce2b1a6d-e550-0410-aec6-3dcde31c8c00