[fir] Add the DoLoopHelper

Add the DoLoopHelper. Some helpers functions
to create fir.do_loop operations.

This code was part of D111337 and was extracted in order to
make the patch easier to review.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D111713



Co-authored-by: Valentin Clement <clementval@gmail.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>