When looking at the performance profile for our new $queryStats workload, I noticed a lot of time spent in __dynamic_cast when computing the shape of {$exists: false} queries which translate into {$not: {$exists: true}}. It was this line, which doesn't seem to need to use dynamic_cast and can use `matchType()` to figure it out instead.

I didn't see any other obvious usages of dynamic_cast in src/mongo/db/matcher's serialization functions.