[llvm] Refactoring llvm-ir2vec.cpp for better separation of concerns in the Tooling classes (PR #170078)

[Mircea Trofin via llvm-commits]

https://github.com/mtrofin commented:

a naming nit, and can you update the title to say it's NFC - it's just a refactoring, correct?

otherwise lgtm

https://github.com/llvm/llvm-project/pull/170078