Improve planning of OR indexscan plans: for quals like

Improve planning of OR indexscan plans: for quals like
	WHERE (a = 1 or a = 2) and b = 42
and an index on (a,b), include the clause b = 42 in the indexquals
generated for each arm of the OR clause.  Essentially this is an index-
driven conversion from CNF to DNF.  Implementation is a bit klugy, but
better than not exploiting the extra quals at all ...