Dept of second thoughts: my first cut at supporting "x IS NOT NULL" btree

Dept of second thoughts: my first cut at supporting "x IS NOT NULL" btree
indexscans would do the wrong thing if index_rescan() was called with a
NULL instead of a new set of scankeys and the index was DESC order,
because sk_strategy would not get flipped a second time.  I think
that those provisions for a NULL argument are dead code now as far as the
core backend goes, but possibly somebody somewhere is still using it.
In any case, this refactoring seems clearer, and it's definitely shorter.