Change behavior of #if ... #then ... #else unification

[ana-pantilie]

Noticed by @virgil-serbanuta: #if ... #then ... #else currently has inconsistent semantics.

In the following example, during function evaluation#if B #then Bottom #else g(x) will be evaluated to Bottom (because a symbol with a Bottom argument is Bottom) while when unifying this term, say unify(stuff, #if B #then Bottom #else g(x) #fi) this will result in ((B == true) and (stuff == Bottom)) or ((B == false) and (stuff == g(x))) therefore the result will be defined (so not Bottom).

@virgil-serbanuta suggests we change this result to ((B == true) and (stuff == f(x)) and (ceil(#if ... #fi))) or ((B == false) and (stuff == g(x)) and (ceil(#if ... #fi))) and investigate any resulting failures.