MIPS: Fix the context check in LoadGlobalFunctionPrototype

Port r18958 (5cd635d0)

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/132883017

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18968 ce2b1a6d-e550-0410-aec6-3dcde31c8c00