[PowerPC] Remove PRED_BAD from PPC::Predicate enumeration.

I'm taking David Blaikie's suggestion to use an
Optional<PPC::Predicate> return value instead.  That's the right
solution for this problem.  Thanks for pointing out that possibility!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183858 91177308-0d34-0410-b5e6-96231b3b80d8