Allow an expression consisting of a single split environment (or other non-numbered AMS environments) to have an automatic equation number when autoNumber is set to 'all'. Also, allow them to have labels if inside a numbered environment (i.e., don't clear the label if there is one).
Resolves issues #2272 (closed) and #1811 (closed).
(The original PR was merged to legacy-v2 instead of legacy-v2-develop, so I reset and am making this new one).