Further cleanup for OR-of-AND WHERE-clauses. orindxpath can now handle

Further cleanup for OR-of-AND WHERE-clauses.  orindxpath can now handle
extracting from an AND subclause just those opclauses that are relevant
for a particular index.  For example, we can now consider using an index
on x to process WHERE (x = 1 AND y = 2) OR (x = 2 AND y = 4) OR ...