Ignore $scopeinfo in write_aiger

While SBY's aiger flow already removes non-assertion driving logic,
there are some uses of write_aiger outside of SBY that could end up with
$scopeinfo cells, so we explicitly ignore them.

The write_btor backend works differently and due to the way it
recursively visits cells, it would never reach isolated cells like
$scopeinfo.

backends/aiger/aiger.cc

@@ -320,6 +320,9 @@ struct AigerWriter
 				continue;
 			}
 
+			if (cell->type == ID($scopeinfo))
+				continue;
+
 			log_error("Unsupported cell type: %s (%s)\n", log_id(cell->type), log_id(cell));
 		}