Remove unneeded parentheses in SQL queries

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#26652 was about where IS NULL was missing parentheses, causing wrong evaluation and incorrect data being returned by queries (x = y IS NULL). This was fixed in a simple way in #26653 for 6.0, but added lots of unneeded parentheses too at the same time.

For 8.0, build upon #23895 to do this better, without adding spurious parentheses.

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Note to self: reenable test Bool_not_equal_nullable_int_HasValue when doing this.