[HWFlattenModules] Add support for verif.formal parent ops.

[dobios]

This is my attempt at adding support for having verif.formal be the parent of an instance to inline.
This currently still doesn't do anything for some reason (but doesn't break the pass).
Any help or ideas are appreciated, particularly if @fabianschuiki has an idea, since I think this is originally your code.

[dobios]

It seems like the problem is that InstanceGraphs can only contain hw::HWModuleOp, I don't feel like it's a good idea to change anything in that since it seems quite load-bearing, maybe creating a more general instance graph that also allows for other "module like" ops could be useful here?

[uenoku]

I think it's possible to add igraph::InstanceGraphOpInterface to FormalOp, but not 100% sure it's going to compose well.