[flang][openacc] Keep region when applying data operand conversion

Similar to D148039 but for the FIR to LLVM IR
conversion pass.

The inner part of the acc.loop has been removed since the rest of the
pipeline is not ready and would raise an error here. This was passing
until now because the acc.loop was discarded completely.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D148057