[FLINK-2814] [optimizer] DualInputPlanNode cannot be cast to SingleInputPlanNode

WorksetIterationNode#instantiate loops over all solution and work set
candidates. Since the solution set reference is modified in place when
the predecessor node can be used in its place, swith this variable to
the inner loop.

This closes #3563

flink-optimizer/src/main/java/org/apache/flink/optimizer/dag/WorksetIterationNode.java

@@ -431,8 +431,8 @@ public class WorksetIterationNode extends TwoInputNode implements IterationNode
 		LocalProperties lp = LocalProperties.EMPTY.addUniqueFields(this.solutionSetKeyFields);
 		
 		// take all combinations of solution set delta and workset plans
-		for (PlanNode solutionSetCandidate : solutionSetDeltaCandidates) {
-			for (PlanNode worksetCandidate : worksetCandidates) {
+		for (PlanNode worksetCandidate : worksetCandidates) {
+			for (PlanNode solutionSetCandidate : solutionSetDeltaCandidates) {
 				// check whether they have the same operator at their latest branching point
 				if (this.singleRoot.areBranchCompatible(solutionSetCandidate, worksetCandidate)) {