Unify the code creating single and multi column semijoin reducers

In HIVE-21196, we add a transformation capable of merging single column semijoin reducers to multi column semijoin reducer.

The code for creating multi-column semijoin reducers in SemiJoinReductionMerge presents some similarities with the code creating single-column semijoin reducers in DynamicPartitionPruningOptimization.

Possibly we could refactor the respective parts to unify the creation logic of semijoin reducers.