[llvm] [SandboxVec] Implement Pass class (PR #107617)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

oh I should have looked at the previous comments

this doesn't really feel like part of SandboxIR but a layer on top that's fairly specific to the vectorizer (I'm guessing most LLVM IR passes that use SandboxIR probably aren't going to have sub-passes that use this pass infra). I would have kept it in SandboxVectorizer, but oh well

https://github.com/llvm/llvm-project/pull/107617