[flang] Add reductions for CUF Kernels: Lowering (#95184)

* Add reductionOperands and reductionAttrs to cuf's KernelOp. 
* Parsing is already working and the tree has the info: here I make the
Bridge emit the updated KernelOp with reduction information added.
* Check |reductionAttrs| = |reductionOperands| in verifier
* Add a test
@clementval @vzakhari

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Co-authored-by: Iman Hosseini <imanh@nvidia.com>
Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>