opt: simplify outer joins in more cases

[DrewKimball]

Describe the problem

Currently, we don't fire null-rejection rules when the query has outer columns. This is to avoid cases where IS NOT NULL filter pushdown rules conflict with decorrelation rules that attempt to pull the filters back up the tree (see #35171).

To Reproduce

The left join in #80901 could not be simplified for the above reason.

Expected behavior

One possibility is to change the way null-rejection rules work; instead of creating an IS NOT NULL filter and relying on filter pushdown rules to ensure it reaches the operator that requested null-rejection, we could traverse the query tree wherever it would be possible to push down an IS NOT NULL filter and simplify outer joins as we go. This would prevent conflicts with decorrelation rules because the simplification would take place 'all at once', without invoking normalization during the traversal.

Jira issue: CRDB-17371

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