[llvm] [GVN] Refactor the LeaderTable structure into a properly encapsulated data structure (PR #88347)

[Owen Anderson via llvm-commits]

resistor wrote:

> Looks reasonable. Is this just a drive-by refactoring or leading up to further changes?

I want to be able to experiment with changes to the underlying data structure, and abstracting it out makes that easier.

So there may be a subsequent change, but I don’t have something to propose currently.

It’s also cleaning up my own mess from many moons ago.

https://github.com/llvm/llvm-project/pull/88347