Method propagateConstant

\brief Replace reads of a given Varnode with a constant. For each read op, check that is in or dominated by a specific block we known the Varnode is constant in. \param varVn is the given Varnode \param constVn is the constant Varnode to replace with \param constBlock is the block which dominates ops reading the constant value \param useMultiequal is \b true if conditional constants can be applie