[enhancement] channel (and func input) flopping should be resolved in a codegen pass

### What's hard to do? (limit 100 words)

Resolution of what type of flopping/buffering to add to ports is currently done in a somewhat ad-hoc manner in block_conversion.cc. This is difficult to test and complicated.

### Current best alternative workaround (limit 100 words)

N/A

### Your view of the "best case XLS enhancement" (limit 100 words)

This resolution should really be done as a resolution pass performed prior to block conversion which would make testing easier and keep everything much more self-contained. As codegen refactor continues this should be one of the things changed.