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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247407 91177308-0d34-0410-b5e6-96231b3b80d8