[llvm] [SandboxVec] Add BottomUpVec test flag to build regions from metadata. (PR #111904)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

> I thought we were going to have a separate function pass for the purposes of testing, rather than hacking this into BoUpVec

I wasn't feeling comfortable with the idea of adding a new pass to the overall list just to test the internals of the vectorizer.

https://github.com/llvm/llvm-project/pull/111904