[flang] Fix the double end do condition. Translate a labelled end do into a

continue and preserve the label for control flow.

Original-commit: flang-compiler/f18@bd611b9809be2dfd70ed310b3a13da7fc45c1742
Reviewed-on: https://github.com/flang-compiler/f18/pull/193
Tree-same-pre-rewrite: false