[mips] Actually mark that the default case is unreachable as this switch
is over a subset of condition codes.

This fixes the -Werror build which warns about use of uninitialized
variables in the default case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219531 91177308-0d34-0410-b5e6-96231b3b80d8