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

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

Chatted offline about moving it to the main SandboxVectorizer pass rather than BottomUpVec. What do you think about this?

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