[flang] Merge pull request flang-compiler/f18#356 from flang-compiler/if-semantics

Implement semantic checks for if statements, if constructs and arithmetic ifs.

Original-commit: flang-compiler/f18@87acf05d0bdd438c768b17bd2e5a893616f04978
Reviewed-on: https://github.com/flang-compiler/f18/pull/356