[flang] Cleanup some cruft and tweaks per review comments.

Added a negative test, to check that semantics are not applied bogusly
to loops that are not DO CONCURRENT.

Original-commit: flang-compiler/f18@d8de45e994de0ff1c4977ded82b1f9918ff83e52
Reviewed-on: https://github.com/flang-compiler/f18/pull/196
Tree-same-pre-rewrite: false