[llvm] [StructuralHash] Refactor (PR #112621)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Mostly looking good.

I also remember running into issues where there would be different hashes between 32-bit and 64-bit x86. Not sure if that's an issue anymore given everything is `uint64_t`, but might be worth testing that the merge functions tests pass.

https://github.com/llvm/llvm-project/pull/112621