[flang] Cleanup code and add test from fir-dev

This patch clean up some code for upstreaming and add couple of
missing tests that were left in fir-dev.

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

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128258



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>