[llvm] [SLP] Compute a shuffle mask for SK_Broadcast shuffle (PR #85327)

[Alexey Bataev via llvm-commits]

================
@@ -7744,16 +7744,22 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
       bool NeedShuffle =
           count(VL, *It) > 1 &&
           (VL.front() != *It || !all_of(VL.drop_front(), UndefValue::classof));
+      if (!NeedShuffle)
+        return TTI.getVectorInstrCost(Instruction::InsertElement, VecTy,
+                                      CostKind, std::distance(VL.begin(), It),
+                                      PoisonValue::get(VecTy), *It);
+
+      SmallVector<int> ShuffleMask(VL.size(), -1);
+      transform(VL, ShuffleMask.begin(), [](Value *V) {
+        return !isa<UndefValue>(V);
----------------
alexey-bataev wrote:

Filling int mask with bools?
```suggestion
        return isa<PoisonValue>(V) ? PoisonMaskElem : 0;   
```

https://github.com/llvm/llvm-project/pull/85327