[Flang] Fixes for XArrayCoorOp

The upstreamed code was not incrementing the sliceOffset in multiples
of 3. This issue is fixed by using Offsets and incrementing by 3 during
every iteration.
In the conversion pattern, we were comparing the definingOp of an
operand with an FIR::UndefOp. Use LLVM::UndefOp for conversion.

Reviewed By: clementval, Leporacanthicus

Differential Revision: https://reviews.llvm.org/D128017