Make PPCInstrInfo::isPredicated always return false

Because of how predication in implemented on PPC (only for branches), I think
that this is the right thing to do.  No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179252 91177308-0d34-0410-b5e6-96231b3b80d8