[llvm] [SandboxVec] Add pass to create Regions from metadata. Generalize SandboxVec pass pipelines. (PR #112288)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

> would be good to get a test in with multiple function passes, perhaps with a null function pass

We don't have a null function pass (we could add it!) but also nothing prevents us from using `-sbvec-passes=bottom-up-vec<>,regions-from-metadata<>`.

I added a test case for it, and it helped me realize (and fix) that I hadn't added a `RegionsFromMetadata::printPipeline` override that prints its region pipeline, so we have that going for us, which is nice :)

https://github.com/llvm/llvm-project/pull/112288