PPC: Don't predicate a diamond with two counter decrements

I've not seen this happen in practice, and probably can't until we start
allowing decrement-counter-based conditional branches to be double predicated,
but just in case, don't allow predication of a diamond in which both sides have
ctr-defining branches. Even though the branching behavior of these can be
predicated, the counter-decrementing behavior cannot be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179199 91177308-0d34-0410-b5e6-96231b3b80d8