Bug fix in code generation.
Bug fix in code generation.

Before this fix, generation for Iff would trigger an infinite loop,
because of automated conversions between Equals for Booleans and Iff.

Includes a regression test for the fixed issue.

epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless