[mips] Added a full set of instruction test cases for all ISA's (but not ASE's).

Summary:
Where those ISA's are not currently supported, the test is run with the smallest
superset of that ISA.

Some instructions are valid but don't pass yet. These have been placed in the
valid-xfail.s's which will XPASS if _any_ instruction starts working.

The valid.s's do not verify the encoding yet. There are also no tests checking that instructions from neighbouring ISA's are not accepted.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3214