typeintersect: fix bounds merging during inner `intersect_all` (#55299)
This PR reverts the optimization from
748149efb3b43e732eab8e9e4276e0ee2bcfa65b (part of #48167), while
keeping the fix for merging occurs_inv/occurs_cov, as that optimzation
makes no sense especially when typevar occurs both inside and outside
the inner intersection.
Close #55206