[flang][fir] handle poly to non poly case in rebox_assumed_rank (#95240)

Dynamic type and element size of the descriptor dummy must match the
dummy static type when the dummy is not polymorphic, otherwise
IS_CONTIGUOUS, C_SIZEOF.... won't work properly inside the callee.

When the actual argument is polymorphic the descriptor of the actual may
have a different dynamic type/element size. Hence, the dummy argument
cannot simply take or copy the descriptor of the actual argument.