MIPS: Remove madd.d usage for r6.

TEST=/benchmarks/octane/crypto
BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/688693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24988 ce2b1a6d-e550-0410-aec6-3dcde31c8c00