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

[via llvm-commits]

vporpo wrote:

> 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

Ideally I think these files should be in a top-level directory like `llvm/SandboxIRPass`. Yeah I doubt there will be other users of these passes, but separating the vectorizer-specific stuff from the rest is a good idea in general.

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