[llvm] [DirectX] Scalarize `extractelement` and `insertelement` with dynamic indices (PR #141676)

[Finn Plummer via llvm-commits]

================
@@ -173,6 +183,125 @@ bool DataScalarizerVisitor::visitStoreInst(StoreInst &SI) {
   return false;
 }
 
+DataScalarizerVisitor::AllocaAndGEPs
+DataScalarizerVisitor::createArrayFromVector(IRBuilder<> &Builder, Value *Vec,
+                                             const Twine &Name = "") {
+  // If there is already an alloca for this vector, return it
+  auto VA = VectorAllocaMap.find(Vec);
+  if (VA != VectorAllocaMap.end())
+    return VA->second;
+
+  auto InsertPoint = Builder.GetInsertPoint();
+
+  // Allocate the array to hold the vector elements
+  Builder.SetInsertPointPastAllocas(Builder.GetInsertBlock()->getParent());
+  Type *ArrTy = equivalentArrayTypeFromVector(Vec->getType());
+  AllocaInst *ArrAlloca =
+      Builder.CreateAlloca(ArrTy, nullptr, Name + ".alloca");
+  const uint64_t ArrNumElems = ArrTy->getArrayNumElements();
----------------
inbelic wrote:

nit:
```suggestion
  const uint64_t ArrNumElems = ArrTy->getArrayNumElements();
  assert(ArrNumElems =< 4 && "For vectors larger than 4, dynamic indexing must be natively supported.");
```

We could also use `Value *[4]` instead of the `SmallVector` if we want to be strict on our assumption above.

https://github.com/llvm/llvm-project/pull/141676