fix unionall typeintersect (#61603)

fix https://github.com/JuliaLang/julia/issues/61602

originated from "real world" example causing unexpected crashes

basically we're getting `intersect_aside(bb->ub, a) =
intersect_aside(Union{Missing, S} where S, Union{Missing, T}) = Any` but
that result is not valid to use in invariant position when RHS contains
a free typevar. so see if we can just use `a` directly.

I think this should also increase parity with the fast-path (which is
hit when there are no free typevars)