[mlir][NVGPU] Fix -Wunsequenced warning

llvm-project/mlir/lib/Conversion/NVGPUToNVVM/NVGPUToNVVM.cpp:441:25:
warning: multiple unsequenced modifications to 'asmArgIdx'
[-Wunsequenced]
  ss << "$" << asmArgIdx++ << ",$" << asmArgIdx++ << ";";
                        ^                      ~~