Relocate and organize reduce then scan building blocks

[danhoeflinger]

          So, there is big amount of new functions are being added in common file `parallel_backend_sycl.h`. Probably, does it make sense to add new headers?

Originally posted by @MikeDvorskiy in #2147 (comment)

Relocate building blocks to reduce then scan header. organize them better and see what redundancy we can remove via refactoring.