Fixes for multirange selectivity estimation
* Fix enumeration of the multirange operators in calc_multirangesel() and
calc_multirangesel() switches.
* Add more regression tests for matching to empty ranges/multiranges.
Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/
c5269c65-f967-77c5-ff7c-
15e621c47f6a%40gmail.com
Author: Alexander Korotkov
Backpatch-through: 14, where multiranges were introduced