lattice: Don't forget to pass down lattice in Conditional tmerge (#47992)
Also add some extra cases that before were taken care of by the
tmerge_fast_path at the entry to the tmerge code. I briefly
considered splitting an extra slow path function to avoid the
one `===` in the ConstsLattice, but in the non-equality case
that check should be quite fast, so it didn't seem worth it.