[flang] Fix issues 518 and 546

- Only allow out-of-scope label target for label-do-stmt
  when the label is on an end-stmt of a scope directly nested
  into the one of the label-do-stmt.
- Also transform label-do-stmt into non-label-do constructs
  so that later phases do not have to handle label-do-stmt.

Original-commit: flang-compiler/f18@91cff2f29698506ac95138107c5f0c625fa237ba
Reviewed-on: https://github.com/flang-compiler/f18/pull/552
Tree-same-pre-rewrite: false