MIPS: Make assert more accurate

Port r16147 (1ced433a)

BUG=

Review URL: https://codereview.chromium.org/22956002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16164 ce2b1a6d-e550-0410-aec6-3dcde31c8c00