[llvm] [LV] Add initial support for vectorizing literal struct return values (PR #109833)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Marking the PR as "ready to review". I think this PR likely still needs further changes, but I don't plan on iterating on this much more without review :slightly_smiling_face: (to be sure this is the right direction).  

https://github.com/llvm/llvm-project/pull/109833