[flang] restructure work in progress in semantics/expression.cc; checkpoint...

[flang] restructure work in progress in semantics/expression.cc; checkpoint before finishing operators

Original-commit: flang-compiler/f18@5d991b0df38937f59aeade66fe413b2b5d4d3a4d
Reviewed-on: https://github.com/flang-compiler/f18/pull/183
Tree-same-pre-rewrite: false