hdl.ir: rework named port handling for Instances.

hdl.ir: rework named port handling for Instances.

The main purpose of this rework is cleanup, to avoid specifying
the direction of input ports in an implicit, ad-hoc way using
the named ports and ports dictionaries.

While working on this I realized that output ports can be connected
to anything that is valid on LHS, so this is now supported too.