Improve patternsel() by applying the operator itself to each value

listed in the column's most-common-values statistics entry.  This gives
us an exact selectivity result for the portion of the column population
represented by the MCV list, which can be a big leg up in accuracy if
that's a large fraction of the population.  The heuristics involving
pattern contents and prefix are applied only to the part of the population
not included in the MCV list.