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

[via llvm-commits]

vporpo wrote:

> didn't notice the first time, but I would have thought this would go in the SandboxVectorizer, not SandboxIR? why are you deciding to put this in SandboxIR?

It was initially in SandboxVectorizer but the Pass and FunctionPass classes are not vectorizer specific so @tschuett suggested we move it to SandboxIR, which sounds reasonable. The Region that the vectorizer will use is specific to the vectorizer, so we can implement that within the SandboxVectorizer directory.

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