Unnecessary RepartitionExec + SortPreservingMergeExec on single-partition sorted output

[neilconway]

Describe the bug

From the plan for TPC-H Q22, in the #21240 branch:

  ScalarSubqueryExec: subqueries=1
    RepartitionExec: partitioning=RoundRobinBatch(4), input_partitions=1, maintains_sort_order=true
      SortPreservingMergeExec: [cntrycode@0 ASC NULLS LAST]
        SortExec: expr=[cntrycode@0 ASC NULLS LAST], preserve_partitioning=[true]
          ...

EnforceDistribution inserts a RepartitionExec: partitioning=RoundRobinBatch(N) on an already-sorted single-partition output, followed immediately by SortPreservingMergeExec to merge them back. This is wasted work — the data is split and immediately re-merged.

This might be related / subset of #4368, not sure exactly.

To Reproduce

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Expected behavior

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Additional context

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[neilconway]

take