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.
The $input_port cell is added by the bufnorm code to simplify handling
of input ports for new code that uses bufnorm, but the aiger2 backend
does already handle input ports separately, so we just ignore those.
With the previous bufnorm implementation inout ports were not supported
at all, so this didn't matter, but with the new bufnorm implementation
they need to be treated as output ports.
`$extern:...` modules inserted by `techmap -extern` are special in the
regard that they have a private ID (starting with a dollar sign) but are
not an internal cell. Support those modules in xaiger export.
- Remove unused statics CONST_FALSE and CONST_TRUE (which appear to have been folded into the `Index` declaration as CFALSE and CTRUE).
- Assign default value of EMPTY_LIT to `a` and `b` for comparison ops.
- Tag debug only variables with YS_MAYBE_UNUSED, don't assign unused variables (but continue to call the function because it moves the file pointer).
