[mlir][openacc][flang] Simplify gang, vector and worker representation (#77667)

The IR representation for gang, vector and worker has grown with the
support for device_type. This patch simplify the IR representation for
gang, vector and worker information on the acc.loop operation.

When the only the keyword is present without any values, the information
is printed at the same place than when there is values. The device_type
is omitted if there is no values and it is equal to None. Otherwise the
full information is displayed. First the keyword only device_type
information and then the values with their device_type.