MatchExpression::getOptimizer() for $or/$and should collapse equivalent clauses

The `$or` operator produces suboptimal results if the parameters in the list are exactly the same, at least when they're empty: `db.collection.count({$or: [ {}, {} ] })`

The way I see it the query planner should be able to see that the parameters are exactly the same and simplify that.

The query `db.collection.count({$or: [ {} ] })` produces a single COUNT stage whereas the more complex query with two empty params creates a SUBPLAN that uses COLLSCAN and only then is able to COUNT so the query is a lot slower.