Fix DefaultIfEmpty lifting logic within SelectMany

[roji]

This fixes the logic which lifts DefaultIfEmpty within the SelectMany collection selector; this optimization causes context.Blogs.SelectMany(b => b.Posts.DefaultIfEmpty() to be translated as a simple LEFT JOIN, instead of having a subquery with the complex DefaultIfEmpty logic.

The previous logic erroneously identified any DefaultIfEmpty anywhere within the SelectMany selector, and lifted it out (translating the SelectMany as LEFT JOIN/OUTER APPLY) in cases which caused incorrect data to be returned:

1. If the DIE is nested within another operator nested in the SelectMany selector, it cannot be lifted out (context.Blogs.SelectMany(b => b.Posts.SelectMany(p => p.Comments.DefaultIfEmpty()). This is #33343.
2. If the DIE appears before another LINQ operator which changes the resultset in non-trivial ways, it also cannot be lifted (e.g. blogs.SelectMany(b => b.Posts.DefaultIfEmpty().Where(p => p.Id > 3))).

We now have state tracking whether DefaultIfEmpty can be lifted as CorrelationFindingExpressionVisitor visits the SelectMany selector.

Fixes #33343
Fixes #19095