[llvm] [GVN] Reorganise GVN.h/GVH.cpp to improve readability and maintainability (NFC) (PR #210334)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I find this change essentially impossible to review. Can you please leave definitions in their old place?

https://github.com/llvm/llvm-project/pull/210334