Add instruction encodings / disassembly support 3r instructions.

It is not possible to distinguish 3r instructions from 2r / rus instructions
using only the fixed bits. Therefore if an instruction doesn't match the
2r / rus format try to decode it as a 3r instruction before returning Fail.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172984 91177308-0d34-0410-b5e6-96231b3b80d8