Mips: Silence a -Wcovered-switch-default

Remove a default label which covered no enumerators, replace it with a
llvm_unreachable.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212729 91177308-0d34-0410-b5e6-96231b3b80d8