Fix cost_mergejoin's failure to adjust for rescanning of non-unique merge join

Fix cost_mergejoin's failure to adjust for rescanning of non-unique merge join
keys when considering a semi or anti join.  This requires estimating the
selectivity of the merge qual as though it were a regular inner join condition.
To allow caching both that and the real outer-join-aware selectivity, split
RestrictInfo.this_selec into two fields.

This fixes one of the problems reported by Kevin Grittner.