The aiger2 backend checks for unsupported cells during indexing. This
causes it to fail when `$connect` or `$tribuf` (as workaround for
missing 'z-$buf support) cells are present in the module.
Since bufnorm adds these cells automatically, it is very easy to end up
with them due to unconnected wires or e.g. `$specify` cells, which do
not pose an actual problem for the backend, since it will never
encounter those during a traversal.
With this, we ignore them during indexing and only produce an actual error
message if we reach such a cell during the traversal.
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