Unused port removal

[rachitnigam]

Add a pass that eliminates unused ports of a component. In doing this, we also get rid of any assignments to those ports and allow the various dead-cell elimination passes to work better.

TODO

• Add a new attribute called @internal or something to state that a component's signature does not need to be maintained to the outside world.
• Implement the pass to remove the unused ports

[sampsyo]

Super dumb question: is this a whole-program analysis? That is, does it need to make a closed-world assumption—that we know all the instantiations of every component, so we can tell that a given port is never used via any of them?

[rachitnigam]

Yup, definitely a whole program analysis

[sampsyo]

Awesome; thanks for clarifying.