[llvm] [LV] Teach LoopVectorizationLegality about struct vector calls (PR #119221)

[Benjamin Maxwell via llvm-commits]

================
@@ -943,11 +955,24 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
       if (CI && !VFDatabase::getMappings(*CI).empty())
         VecCallVariantsFound = true;
 
+      auto canWidenInstruction = [this](Instruction const &Inst) {
+        Type *InstTy = Inst.getType();
+        if (isa<CallInst>(Inst) && isa<StructType>(InstTy) &&
+            canWidenCallReturnType(InstTy)) {
+          StructVecVecCallFound = true;
----------------
MacDue wrote:

I don't want loop `LoopVectorizationLegality` to fail. The intention is `LoopVectorizationLegality` sees these calls are vectorizable (and returns normally). It's just the LoopVectorizer is incomplete and can't handle them yet. I think @david-arm has done something similar in the past. 

This flag is meant to be easily removed once it's supported. 


https://github.com/llvm/llvm-project/pull/119221