[FIRRTL] Enable access to IMDCE at smaller granularities

[seldridge]

I've turned on IMDCE in circt-reduce for FIRRTL. However, this is a very heavy hammer. E.g., I have a circuit with dead code. However, when I run it through IMDCE it deletes essentially everything. In order for this to be effective in the reducer, there needs to be a more targeted way of applying this.

There are a couple of features that would help with this:

1. Being able to run IMDCE on a per-module basis.
2. Being able to run IMDCE on only specific signals. E.g., on a per-port basis.

Likely this would entail breaking the main IMDCE pass into smaller units (or utilities) that could be reused.

There are alternatives here which involve selective application of inner symbols to block removal. However, that's a bit rough.

h/t @uenoku for the idea.