Avoid O(N^2) processing in X in [set1] | x in [set2]

There is some code to optimize 

x in [a,b,c] or x in [c, d, e]

to

x in [a,b,c,d,e]

but the implementation in O(N^2) in the length of the list, which can lead to significant time being taken for large sets.

Solution is to limit to small sets (where most of the advantage lies) and also ensure only constant sets (which was the intention).