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

[Benjamin Maxwell via llvm-commits]

================
@@ -946,8 +946,8 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
       // Check that the instruction return type is vectorizable.
       // We can't vectorize casts from vector type to scalar type.
       // Also, we can't vectorize extractelement instructions.
-      if ((!VectorType::isValidElementType(I.getType()) &&
-           !I.getType()->isVoidTy()) ||
+      Type *InstTy = I.getType();
+      if (!(InstTy->isVoidTy() || canWidenType(InstTy)) ||
----------------
MacDue wrote:

I've added two extra tests with `load` and `store` instructions, and updated the check in `LoopVectorizationLegality.cpp` to only allow widening calls that return structs, where all users are `extractvalue`. 

This is all we need for this initial patch (we could maybe generalize this more in future patches). 

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