Implement @samarion's fix for mergeCalls code explosion
Created by: jad-hamza
Fix https://github.com/epfl-lara/inox/issues/146
I changed the assignment to: https://github.com/epfl-lara/inox/compare/master...jad-hamza:merge-calls?expand=1#diff-23a8e9c2ed0ae3fba86af88fb35feb7a7b31d1d7b92e6caa2f4f2ba9f1f5164fR129-R132