[mlir][flang] Make use of the new `GEPArg` builder of GEP Op to simplify code

This is the follow up on https://reviews.llvm.org/D130730 which goes through upstream code and removes creating constant values in favour of using the constant indices in GEP directly. This leads to less and more readable code and more compact IR as well.

Differential Revision: https://reviews.llvm.org/D130731