[flang] Fix XArrayCoorOp conversion for index type slices

The previous XArrayCoorOp conversion forgot to change getting the
operands from OpAdaptor for upper bound and step of slice. This leads to
the fail of incompatible of types of codegen when slices are index type.

Reviewed By: kiranchandramohan, schweitz

Differential Revision: https://reviews.llvm.org/D125967