Make CommonSubexprEliminate top-down like (#11683)

* Make `CommonSubexprEliminate` top-down like

* fix top-down recursion, fix unit tests to use real a Optimizer to verify behavior on plans

* Extract result of `find_common_exprs` into a struct (#4)

* Extract the result of find_common_exprs into a struct

* Make naming consistent

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Co-authored-by: Andrew Lamb <[email protected]>

Author: peter-toth