Structural extraction: Avoid a potential ambiguity in the definition

of an If-Then-Else node. This change requires the "then" and "else"
branches to share the same successors (which can be either either
*a single common successor* or *no successor at all*).

This does not currently affect the output in any significant way,
but it is preferable to squash any latent nondeterminism in the
algorithm.

Bug: #51