Show links for replaced nodes in loops

[laike9m]

When nodes in a loop are replaced, the lines involving replaced nodes disappear. I believe even if these edges are not accurate after modifying loop counters, they still provide useful information thus should be kept.

We can use dotted lines for this, but we need to add a caption or tooltip to show what it means.

[laike9m]

We should do it after #108 is fixed.