replaced dyn_cast_or_null with dyn_cast in UsedVariablesFinder
replaced dyn_cast_or_null with dyn_cast in UsedVariablesFinder

rafalprzywarski/libcppmanip,rafalprzywarski/libcppmanip,rafalprzywarski/libcppmanip,rafalprzywarski/libcppmanip,rafalprzywarski/libcppmanip