that blocked building index merge plans for the queries with where
conditions of the form 
(key1|2_p1=c AND range(key1_p2)) OR (key1|2_p1=c AND range(key2_p2)).

The problem was discovered by Sergey Petrunia when he reviewed the patch
for WL#24.

Added new test cases. One of them failed to produce an index merge plan
before the patch was applied.