pgsql: Improve patternsel() by applying the operator itself to each

Log Message:
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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.

Modified Files:
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    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.194 -> r1.195)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.194&r2=1.195)