[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!

llvm-svn: 183858

llvm/lib/Target/PowerPC/MCTargetDesc/PPCPredicates.h

@@ -32,8 +32,7 @@ namespace PPC {
     PRED_GT     = (1 << 5) | 12,
     PRED_NE     = (2 << 5) |  4,
     PRED_UN     = (3 << 5) | 12,
-    PRED_NU     = (3 << 5) |  4,
-    PRED_BAD    = 0
+    PRED_NU     = (3 << 5) |  4
   };
   
   /// Invert the specified predicate.  != -> ==, < -> >=.