[flang] Combined the implementations to ```Pre()``` and ```Post()``` functions that

call `PushConstruct()``` and ```PopConstruct()``` following a genius
suggestion from Peter.

Original-commit: flang-compiler/f18@be2a03ebf479be8eda2529e6c47a7d44a073c455
Reviewed-on: https://github.com/flang-compiler/f18/pull/686