[flang][OpenMP] Fix parallel do with integer*1 var

Temporaries created to store worksharing loop index values were
using different types than that of the original index variables.
This caused invalid IR to be produced when an index variable was
used in binary operations which expected its original type.

Fix this by creating temporaries with the types of their original
variables and converting the loop values, that continue to use
the types that OpenMP runtime expects, to them.

Fixes https://github.com/llvm/llvm-project/issues/60870

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D156803