Consider removing subquery pushdown in preprocessing (SubqueryMemberPushdownExpressionVisitor)

[roji]

For this test:

public virtual Task Index_constant(bool async)
    => AssertQuery(
        async,
        ss => ss.Set<RootEntity>().Where(e => e.RelatedCollection[0].Int == 21),
        ss => ss.Set<RootEntity>().Where(e => e.RelatedCollection.Count > 0 && e.RelatedCollection[0].Int == 21));

In preprocessing, before SubqueryMemberPushdownExpressionVisitor we have the following tree:

DbSet<RootEntity>()
    .Where(e => e.RelatedCollection
        .AsQueryable()
        .ElementAt(0).Int == 21)

Afterwards we have:

DbSet<RootEntity>()
    .Where(e => e.RelatedCollection
        .AsQueryable()
        .Select(s => s.Int)
        .ElementAt(0) == 21)

In other words, the member access for Int gets "pushed down" to before the ElementAt() call, and transformed to a Select(). For some context, see #30386 (comment).

Trouble is, this makes the tree complicated to process for e.g. simple complex translation: the original tree corresponds very well to simple JSON drilling into the document, where as the transformed one does not, and requires later hackery in the query pipeilne.

See also #30386, where this caused issues in EFCore.PG.