[flang][OpenMP] Fix the types of worksharing-loop variables

The types of lower bound, upper bound, and step are converted into the
type of the loop variable if necessary. OpenMP runtime requires 32-bit
or 64-bit loop variables. OpenMP loop iteration variable cannot have
more than 64 bits size and will be narrowed.

This patch is part of upstreaming code from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project

. (#1256)

Co-authored-by: kiranchandramohan <kiranchandramohan@gmail.com>

Reviewed By: kiranchandramohan, shraiysh

Differential Revision: https://reviews.llvm.org/D125740