Method findLoopVariable

Try to find a Varnode that represents the controlling \e loop \e variable for \b this loop. The Varnode must be: - tested by the exit condition - have a MULTIEQUAL in the head block - have a modification coming in from the tail block - the modification must be the last op or moveable to the last op If the loop variable is found, this routine sets the \e iterateOp and the \e loopDef. \param cbranc