[mips] Add Octeon cnMips instructions mtmX and mtpX

Adds the Octeon cnMips instructions "load multiplier register MPLx" and "load product register Px".
Includes tests.

Reviews by: Daniel.Sanders@imgtec.com

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205343 91177308-0d34-0410-b5e6-96231b3b80d8