In the push-down optimization, do not substitute columns that have previously

In the push-down optimization, do not substitute columns that have previously
been identified as being constant by the propagate-constants optimization.
Fix for ticket [51166be0159fd2ce].
Also, avoid factoring out constant expressions on the LHS of an IN(...)
operator, as the IN(...) operation may affect the affinity of these values.
Fix for [fd1bda016d1].

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