get_actual_variable_range() hasn't used a dirty snapshot since commit
3ca930fc, which invented a new snapshot type specifically to meet
selfuncs.c's requirements (HeapTupleSatisfiesNonVacuumable() type
snapshots were added).

Discussion: https://postgr.es/m/CAH2-Wzn2pSqEOcBDAA40CnO82oEy-EOpE2bNh_XL_cfFoA86jw@mail.gmail.com