[mlir][openacc] Add conversion for if operand to scf.if for standalone data operation

This patch convert the if condition on standalone data operation such as acc.update,
acc.enter_data and acc.exit_data to a scf.if with the operation in the if region.
It removes the operation when the if condition is constant and false. It removes the
the condition if it is contant and true.

Conversion to scf.if is done in order to use the translation to LLVM IR dialect out of the box.
Not sure this is the best approach or we should perform this during the translation from OpenACC
to LLVM IR dialect. Any thoughts welcome.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D103325