[Mips Disassembler] Have the DecodeCCRRegisterClass function use the getReg
function to lookup the proper tablegen'ed register enumeration.  Previously,
it was using the encoded value directly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185026 91177308-0d34-0410-b5e6-96231b3b80d8