public ExprWalkerInfo getPrunedPreds(Operator<? extends OperatorDesc> op) {
    return opToPushdownPredMap.get(op);
  }
Operator to Pushdown Predicates Map. This keeps track of the final pushdown
predicates for each operator as you walk the Op Graph from child to parent
ql/src/java/org/apache/hadoop/hive/ql/ppd/OpWalkerInfo.java