[mips] Add more Octeon cnMips instructions

Adds the instructions ext/ext32/cins/cins32.
It also changes pop/dpop to accept the two operand version and
adds a simple pattern to generate baddu.
Tests for the two operand versions (including baddu/dmul/dpop/pop)
and the code generation pattern for baddu are included.

Reviewed by: Daniel.Sanders@imgtec.com