MIPS: Avoid redundant smi check for Math.abs

Port r16021 (d79f4450)

BUG=

Review URL: https://codereview.chromium.org/21903003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16035 ce2b1a6d-e550-0410-aec6-3dcde31c8c00