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

[Farzon Lotfi via llvm-commits]

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@@ -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);
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farzonl wrote:

nit: i would prefer if the name would indicate we are using an iterator,
Also maybe we should do a more modern way like
```cpp
if (auto it = VectorAllocaMap.find(Vec); it != VectorAllocaMap.end()) {
    return it->second; 
}
```
 or maybe something using contains `if (VectorAllocaMap.contains(Vec)) {`

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