[llvm] [SLP] Compute a shuffle mask for getGatherCost (PR #85330)

[Alexey Bataev via llvm-commits]

================
@@ -10516,27 +10516,34 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL,
           TTI->getVectorInstrCost(Instruction::InsertElement, VecTy, CostKind,
                                   I, Constant::getNullValue(VecTy), V);
   };
+  SmallVector<int> ShuffleMask(VL.size(), PoisonMaskElem);
   for (unsigned I = 0, E = VL.size(); I < E; ++I) {
     Value *V = VL[I];
     // No need to shuffle duplicates for constants.
     if ((ForPoisonSrc && isConstant(V)) || isa<UndefValue>(V)) {
       ShuffledElements.setBit(I);
+      ShuffleMask[I] = I;
----------------
alexey-bataev wrote:

```suggestion
      ShuffleMask[I] = isa<PoisonValue>(V) ? PoisonMaskElem : I;
```

https://github.com/llvm/llvm-project/pull/85330