[llvm] Reapply "[SandboxIR][NFC] Move Region from SandboxVectorizer to SandboxIR." (#110173) (PR #110181)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

The windows checks have passed, and the linux ones seem unrelated (libc++ test failures). The original PR had already been reviewed, so I'm merging this now.

https://github.com/llvm/llvm-project/pull/110181