Re-commit r247405: [mips] Add missing MIPS-I disassembler tests.

These tests were found by llvm-mc-fuzzer (see http://reviews.llvm.org/D12723)
and verified by checking the disassembler output is accepted by GAS.

The problematic tests from the previous commit have been moved to
valid-xfail.txt for now.

Also, give invalid instructions some coverage. invalid-xfail.txt contains
instructions that should be invalid but successfully disassemble.

llvm-svn: 247407