[MIPS] Add cpu octeon and some instructions

[MIPS] Add cpu octeon and some instructions

The Octeon cpu from Cavium Networks is mips64r2 based and has an extended
instruction set. In order to utilize this with LLVM, a new cpu feature "octeon"
and a subtarget feature "cnmips" is added. A small set of new instructions
(baddu, dmul, pop, dpop, seq, sne) is also added. LLVM generates dmul, pop and
dpop instructions with option -mcpu=octeon or -mattr=+cnmips.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204337 91177308-0d34-0410-b5e6-96231b3b80d8