[flang] Fix couple of issue with user defined assignment in FORALL and WHERE

This patch fixes a couple of issues with the lowering of user defined assignment.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D128730



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>