[flang] Merge pull request flang-compiler/f18#320 from flang-compiler/pmk-expr-rearrangement

Rearrange/simplify expr analysis code

Original-commit: flang-compiler/f18@4eaccde5167370eb082ce6ef0247942a230f94f1
Reviewed-on: https://github.com/flang-compiler/f18/pull/320