[flang] FIR statements: various reworking of the code to decompose some

[flang] FIR statements: various reworking of the code to decompose some
operations, get rid of unneeded nodes, restructure DO loops, etc.

Original-commit: flang-compiler/f18@0e0d67fbdae307b63f31ade0a990db8f00fed251
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false