[llvm] [SandboxVec] Add barebones Region class. (PR #108899)

[via llvm-commits]

https://github.com/vporpo commented:

Please add a unit test `llvm/unittests/Transforms/Vectorize/SandboxVectorizer/RegionTest.cpp` to test the API.

Also, regarding the class name, any thoughts on a more suitable name?

https://github.com/llvm/llvm-project/pull/108899