rv/rvgen: fix isinstance check in Variable.expand()

The Variable.expand() method in ltl2ba.py performs contradiction
detection by checking if a negated variable already exists in the
graph node's old set. However, the isinstance check was incorrectly
testing the ASTNode wrapper instead of the wrapped operator, causing
the check to always return False.

The old set contains ASTNode instances which wrap LTL operators via
their .op attribute. The fix changes isinstance(f, NotOp) to
isinstance(f.op, NotOp) to correctly examine the wrapped operator
type. This follows the established pattern used elsewhere in the
file, such as the iteration at lines 572-574 which accesses
o.op.is_temporal() on items from node.old.

Signed-off-by: Wander Lairson Costa <[email protected]>
Reviewed-by: Nam Cao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gabriele Monaco <[email protected]>

tools/verification/rvgen/rvgen/ltl2ba.py

@@ -395,7 +395,7 @@ class Variable:
     @staticmethod
     def expand(n: ASTNode, node: GraphNode, node_set) -> set[GraphNode]:
         for f in node.old:
-            if isinstance(f, NotOp) and f.op.child is n:
+            if isinstance(f.op, NotOp) and f.op.child is n:
                 return node_set
         node.old |= {n}
         return node.expand(node_set)