mips64: VEX support for MIPS64 Octeon Instructions

Modeling the following Cavium Octeon instructions:

exts, exts32, cins, cins32, snei, sne, seqi, dmul, seq.

Patch by Zahid Anwar.

Bugzilla issue 322150


git-svn-id: svn://svn.valgrind.org/vex/trunk@2765 8f6e269a-dfd6-0310-a8e1-e2731360e62c