[circle-opselector] Support multiple subgraphs

[seanshpark]

Current circle-opselector supports only single subgraph model.
There is a case where we want to extract the first subgraph without nodes that are related with other subgraphs like If or While.

Condition of selection need not be changed; select nodes only from first subgraph.

• if If or While is inside selection, throw error

@mhs4670go , can we support this feature?

[seanshpark]

done for now