SQL: Update CaseToCoalesceRule to ignore irrelevant casts.

[gianm]

Previously, CaseToCoalesceRule ignored nullability casts when deciding if "x" and "y" in "CASE WHEN x IS NOT NULL THEN y" match and could therefore form the basis of a COALESCE. This was too strict, since it considered a cast from VARCHAR to VARCHAR(2000), which can happen after applying JSON_VALUE, to be relevant. It is not relevant, because it does not affect how we execute the query.

This patch updates the logic to ignore nullability casts and also casts that change irrelevant properties of the type. ("Irrelevant" being defined as "properties that do not affect how the query executes".)

This change speeds up queries, because generally COALESCE runs faster than CASE. Note that one of the test cases is now vectorizable, where it wasn't previously. The new tests are also both vectorizable.

[github-actions[bot]]

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