MIPS: Fix implementation of string slices.
MIPS: Fix implementation of string slices.

Bug on mips using at register innapropriately, use different reg as temp.

BUG=
TEST=

Review URL: http://codereview.chromium.org/7794020

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@9071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh