mir: use `FiniteBitSet<u32>` in polymorphization

mir: use `FiniteBitSet<u32>` in polymorphization

This commit changes polymorphization to return a `FiniteBitSet<u32>`
rather than a `FiniteBitSet<u64>` because most functions do not use
anywhere near sixty-four generic parameters so keeping a `u64` around is
unnecessary in most cases.

Signed-off-by: David Wood <david@davidtw.co>