[llvm] [SandboxVectorizer] Use sbvec-passes flag to create a pipeline of Region passes after BottomUpVec. (PR #111223)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

> yeah, all the non-owning pointers make me sus

Got rid of the non-owning pointer. Now the region pass manager is a simple field of `BottomUpVec`.

https://github.com/llvm/llvm-project/pull/111223