planner: treat plans with different in-list length as the same one in plan cache

[qw4990]

Enhancement

Currently, for 2 'same' queries with different in-list lengths, plan cache will generate and cache different plans for them.
For example, plan cache will cache 2 plans for the 2 'same' queries below:
select * from t where a in (1, 2), select * from t where a in (1, 2, 3).

This strategy can cause lots of memory usage:

[fixdb]

I think this has been addressed on master branch.