Add artificial PRED_BAD to PPC::Predicate enumeration.

Allows returning a PPC::Predicate from a function with a no-predicate
value possible.  Preparatory patch for fast-isel on PPC64 ELF.  No
behavioral change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183841 91177308-0d34-0410-b5e6-96231b3b80d8