(Minor) handle corner case in conditional branch API

[sanha]

If we union two conditional branches right after the branching, during the optimization, the conditional branch vertex should have two edges toward the union operator. However, this cannot be represented as a DAG that our system supports. Fortunately, this situation is very weird and minor corner case. Anyway, we have to deal with this (at least throw an exception).

• when constructing logical DAG

• during the DAG optimization