[flang] Merge pull request flang-compiler/f18#203 from flang-compiler/eas7

do a little polishing on the DO loop canonicalization code

Original-commit: flang-compiler/f18@93e68870530184fc0d7c0ce0584b956b0fbf373e
Reviewed-on: https://github.com/flang-compiler/f18/pull/203