opt_clean, simplemap: Add $buf handling

[povik]

What are the reasons/motivation for this change?

The write_xaiger2 command transiently enters the buffered-normalized mode. This can introduce $buf cells into the design. These cells are missing handling by the opt_clean and techmap passes, and as such they just fall through to the end of a flow.

So far the bufnormed mode usage has been isolated to the insides of a single pass, once we will want to extend this to let designs in bufnormed mode cross from one pass to another, we will need to revisit the opt_clean/techmap behavior. At that point we will need to revisit all other cases where these passes introduce connections anyway.

If applicable, please suggest to reviewers how they can test the change.

There's a minimal buf.ys test of the new behavior