[openacc] Update acc.loop to expose data operands (#70954)

The compute and data constructs implement getNumDataOperands and
getDataOperand. The acc.loop operation similarly has multiple data
operands - thus it makes sense to expose them the same way.

For loop, only private and reduction operands are exposed this way.
Technically, acc.loop also holds cache operands - but these are hints
not a data attribute.