Tweak sorting so that nulls appear at the front of a descending sort

(vs. at the end of a normal sort).  This ensures that explicit sorts
yield the same ordering as a btree index scan.  To be really sure that
that equivalence holds, we use the btree entries in pg_amop to decide
whether we are looking at a '<' or '>' operator.  For a sort operator
that has no btree association, we put the nulls at the front if the
operator is named '>' ... pretty grotty, but it does the right thing in
simple ASC and DESC cases, and at least there's no possibility of getting
a different answer depending on the plan type chosen.