Removing potentially error-prone fallthrough. NFC

This fallthrough if other cases are added between fabs and default
could cause fabs to fall to the next case resulting in a bug.
Better getting rid of it immediately just to be sure.

llvm-svn: 292003

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4267,6 +4267,7 @@ static Value *SimplifyIntrinsic(Function *F, IterTy ArgBegin, IterTy ArgEnd,
     case Intrinsic::fabs: {
       if (SignBitMustBeZero(*ArgBegin, Q.TLI))
         return *ArgBegin;
+      return nullptr;
     }
     default:
       return nullptr;