* we search for each use all definitions in the current block, if none is
* found, then we search one in the immediate dominator. If we are in a block
* of the dominance frontier, create a phi and do the same search for all
* we search for each use all definitions in the current block, if none is
* found, then we search one in the immediate dominator. If we are in a block
* of the dominance frontier, create a phi and do the same search for all