We were performing the helper passes `abc9_ops -replace_zbufs` and
`abc9_ops -restore_zbufs` for every module, but those passes act on the
full design (and can't be applied entirely selectively due to entering
and leaving bufnorm).
This lead to an explosive creation of a lot of redundant bufnorm helper
cells that would have been cleaned up by `clean` but that never ran.
Instead we now run each helper pass once, one before and one after
iterating over the selected modules. This limits the number of bufnorm
helper cells.
With the updated bufnorm code, buffered 'z drivers are used as anchor
points for undirected connections. These are currently not supported by
read/write_xaiger2, so we temporarily replace those by roughly
equivalent $tribuf cells which will be handled as blackboxes that
properly roundtrip through the xaiger2 front and backend.
Previously the `abc9_box` mode was reserved to modules with the
`blackbox` or `whitebox` attribute. Allow `abc9_box` on ordinary modules
when doing hierarchical synthesis.
