MIPS: Remove ASSERT since there are tons of different ASSIGN variants

Port r22382 (84a841f)

BUG=
R=akos.palfi@imgtec.com

Review URL: https://codereview.chromium.org/395503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22386 ce2b1a6d-e550-0410-aec6-3dcde31c8c00