Improve "like-types" collapsing

[matthchr]

Fix hacky change applied in #4430. Instead of this, create a new stage that can merge "like" types within a group.

Doing this may require comparing a bunch of types. We may need to do some optimization such as pre-computing the number of properties on objectTypes and only comparing ObjectTypes that have the same number of properties if it's really slow.
Alternatively, only looking for types that match a certain type (so it's O(N) rather than O(N^2).