Improving struct pruning

[robotrobo]

Continues work from issue #5 , follows from PR #14.

[robotrobo]

I was trying to improve the ListStructs inspector and found this :-
Lets say we have a struct abcd which has three fields initially. Now, we have a method call in our p4 program,
methodA({0w10,0w1,0w2}) where methodA takes an argument of type abcd, but, our pruner removes all the fields of abcd because they are not accessed anywhere else. So, how do we tell ListStructs that this ListExpression tuple will be the initializerList for a particular struct. Inside IR::MethodCallExpression, there is a vector called typeArguments, but they currently seem to be empty. Maybe we will need to populate the typeMap?

[fruffy]

Usually, you can tell the type of the structure by looking up the method declaration. So in this case, you would look up the declaration of methodA and check what the type of the parameter is. We can use the refmap and typeChecking for this if you want to. With the refMap and the typeMap you should be able to look up such declarations.