[Flang] Add flag dependent code to execute the loop-body atleast once

Given the flag `--always-execute-loop-body` the compiler emits code
to execute the body of the loop atleast once.

Note: This is part of upstreaming from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project.

Reviewed By: awarzynski, schweitz

Differential Revision: https://reviews.llvm.org/D127128



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>
Co-authored-by: Sameeran Joshi <sameeranjayant.joshi@amd.com>