[flang][openacc] Use original input for base address with optional (#80931)

In #80317 the data op generation was updated to use correctly the #0
result from the hlfir.delcare op. In case of optional that are not
descriptor, it is preferable to use the original input for the varPtr
value of the OpenACC data op.
This patch also make sure that the descriptor value of optional is only
accessed when present.