MIPS: Elide unnecessary context reload in generated stubs.

Port r13290 (6970ca83)

BUG=
TEST=

Review URL: https://codereview.chromium.org/11773013
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13323 ce2b1a6d-e550-0410-aec6-3dcde31c8c00