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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

I've created a new draft PR for the multiple-defs implementation here: https://github.com/llvm/llvm-project/pull/115928. This version passes the same tests as this PR and works by making both `VPWidenCallRecipe` and `VPReplicateRecipe` return multiple values for struct types. I have commented to point out several transforms that don't yet support multiple defs (and don't seem trivial to update) but needed some changes to allow the single-def case (to get tests passing again).

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