Move processing each part of the query into its own functions

This commit doesn't change any of the logic at all.

Instead, the goal is to:

 * Get rid of any code duplication
 * Incremental changes to the optimizer made it slightly hard
   to follow the code, improve that and make it easier to
   implement new features
 * Simplify the code by moving each part of query processing (e.g.,
   DISTINCT, LIMIT etc) into its own function
 * Make the interaction between each part of the query more
   obvious (e.g., How DISTINCT affects LIMIT etc)

src/backend/distributed/planner/extended_op_node_utils.c

@@ -123,6 +123,10 @@ GroupedByDisjointPartitionColumn(List *tableNodeList, MultiExtendedOp *opNode)
 }
 
 
+/*
+ * ExtendedOpNodeContainsRepartitionSubquery is a utility function that
+ * returns true if the extended op node contains a re-partition subquery.
+ */
 static bool
 ExtendedOpNodeContainsRepartitionSubquery(MultiExtendedOp *originalOpNode)
 {