Remove PPCScoreboardHazardRecognizer

PPCScoreboardHazardRecognizer was a subclass of ScoreboardHazardRecognizer
which did only one thing: filtered out nodes in EmitInstruction for which
DAG->getInstrDesc(SU) returned NULL. This used to be the case for PPC pseudo
instructions. As far as I can tell, this is no longer true, and so we can use
ScoreboardHazardRecognizer directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196171 91177308-0d34-0410-b5e6-96231b3b80d8