mips64: add tests for MIPS64 Octeon Instructions

Follow up to VEX r2765.

Patch by Zahid Anwar.

Bugzilla issue 322150


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13558 a5019735-40e9-0310-863c-91ae7b9d1cf9