Add a pass for minimizing recursion groups (#6832)

Most of our type optimization passes emit all non-public types as a
single large rec group, which trivially ensures that different types
remain different, even if they are optimized to have the same structure.
Usually emitting a single large rec group is fine, but it also means
that if the module is split, all of the types will need to be repeated
in all of the split modules. To better support this use case, add a pass
that can split the large rec group back into minimal rec groups, taking
care to preserve separate type identities by emitting different
permutations of the same group where possible or by inserting unused
brand types to differentiate them.

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