tree: OPERATOR pretty printing changes

[jseldess]

PR: cockroachdb/cockroach#68041

From release notes:

Previously, pretty printing could fold some OPERATOR expressions based on the order of operations of the operator inside the OPERATOR. This can lead to a different order of operations when reparsing, so this is fixed by never folding OPERATOR expressions. [#68041][#68041] {% comment %}doc{% endcomment %}