Set VarNodes and FieldNodes in InfoCollector

in InfoCollector, there is a step which collects violated variables and fields.
In the phase, it decide the main node of graph.
Therefore, we have to check the node is really core node.

Start by reading the InfoCollector phase that collects violated variables and fields and determines the graph's main node. Trace how VarNodes and FieldNodes are currently handled, then define and verify the condition that identifies a core node. Done means the collector sets these nodes only after confirming they are truly core nodes.

Written by the indexing model from the issue text.