[fir] Add cfg conversion pass

This patch upstream the cfg conversion pass. This pass
rewrite FIR loop-like operation to a CFG.

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

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D111095