pgsql: Generalize mcv_selectivity() to support both VAR OP CONST and

Log Message:
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Generalize mcv_selectivity() to support both VAR OP CONST and CONST OP VAR
cases.  This was not needed in the existing uses within selfuncs.c, but if
we're gonna export it for general use, the extra generality seems helpful.
Motivated by looking at ltree example.

Modified Files:
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    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.202 -> r1.203)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.202&r2=1.203)
    pgsql/src/include/utils:
        selfuncs.h (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h.diff?r1=1.31&r2=1.32)