[llvm] [IRBuilder][ConstantFold] Fold constant for CreateInsertVector. (PR #116229)

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

> There shouldn't be an IRBuilderFolder API that is specific to this intrinsic. This is a specific instance of general intrinsic folding support.

What should I do if I want to make `CreateInsertVector` fold constant just like `CreateInsertElement`
```
  Value *CreateInsertElement(Value *Vec, Value *NewElt, Value *Idx,
                             const Twine &Name = "") {
    if (Value *V = Folder.FoldInsertElement(Vec, NewElt, Idx))
      return V;
    return Insert(InsertElementInst::Create(Vec, NewElt, Idx), Name);
  }
```

https://github.com/llvm/llvm-project/pull/116229