[Flang] Lower the infinite do loop

The basic infinite loop is lowered to a branch to the body of the
loop, and the body containing a back edge as its terminator.

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

Reviewed By: rovka

Differential Revision: https://reviews.llvm.org/D126697



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