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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> What would be the best way to test this on some larger code? Is there a vector library that already defines some functions with multiple return values (or some follow-up patches)?

There's a follow-up patch https://github.com/llvm/llvm-project/pull/123210 to enable vectorizing the `llvm.sincos` intrinsic (which can be lowered to from clang's `__builtin_sincos()`), and we intend to do similar patches for a few other intrinsics. 

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