PATCH: add support for IN and @> in functional-dependency statistics use

Hello

At my current job, we have a lot of multi-tenant databases, thus with tables 
containing a tenant_id column. Such a column introduces a severe bias in 
statistics estimation since any other FK in the next columns is very likely to 
have a functional dependency on the tenant id. We found several queries where 
this functional dependency messed up the estimations so much the optimizer 
chose wrong plans.
When we tried to use extended statistics with CREATE STATISTIC on tenant_id, 
other_id, we noticed that the current implementation for detecting functional 
dependency lacks two features (at least in our use case):
- support for IN clauses
- support for the array contains operator (that could be considered as a 
special case of IN)

After digging in the source code, I think the lack of support for IN clauses 
is an oversight and due to the fact that IN clauses are ScalarArrayOpExpr 
instead of OpExpr. The attached patch fixes this by simply copying the code-
path for OpExpr and changing the type name. It compiles and the results are 
correct, with a dependency being correctly taken into consideration when 
estimating rows. If you think such a copy paste is bad and should be factored 
in another static bool function, please say so and I will happily provide an 
updated patch.
The lack of support for @> operator, on the other hand, seems to be a decision 
taken when writing the initial code, but I can not find any mathematical nor 
technical reason for it. The current code restricts dependency calculation to 
the = operator, obviously because inequality operators are not going to 
work... but array contains is just several = operators grouped, thus the same 
for the dependency calculation. The second patch refactors the operator check 
in order to also include array contains.

I tested the patches on current HEAD, but I can test and provide back-ported 
versions of the patch for other versions if needed (this code path hardly 
changed since its introduction in 10).

Best regards

 Pierre Ducroquet