Variable length sarging for OR predicates

Allow variable length sarging for OR predicates.

CREATE INDEX idx_ab ON default( a, b );

SELECT *

FROM default

WHERE a = 5 OR ( a = 10 AND b = 10 );

This will produce two spans:
[ a: 5 ]
[ a: 10 ]

It should instead produce two variable-length spans:
[ a: 5 ]
[ a: 10, b: 10 ]