[reggen] Check for unused regwen

We should add the ability to check for "dangling" regwen, ie, regwen's that are declared but not used.
At the moment, if we have situations like this, it's not clear it would be catch by DV's automated testing, since I believe DV checks for correct behavior for regwen's that are actually used.

Start by locating the reggen entry point and reading how regwen declarations and uses are currently represented. Trace the existing checks and DV coverage, then define the unused-regwen diagnostic so a declaration with no use is reported and covered by a regression test.

Written by the indexing model from the issue text.