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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> This seems reasonable in general, although would be good if possible to avoid too many changes, possibly by keeping `VPRecipeIRFlags` as single def for now, moving the IR flags to a separate class with `VPRecipeIRFlags` inheriting from both `VPSingleDefRecipe` & the class to manage the flags?

That is pretty much what I did in #112402, but I called that class `VPSingleDefRecipeWithIRFlags`. If this is the approach we want to use, I think I can split that out into its own patch (which is pretty much an NFC)

> Would it be possible to get an initial version working without support for replicate recipes?

Maybe, I think the minimum support I need is this working for both `VPWidenCallRecipe` and `VPWidenIntrinsicRecipe`.
Though `VPWidenIntrinsicRecipe` may be able to wait for a follow-up patch (as it'll need additional cost model changes).





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