[llvm] [SLP] Compute a shuffle mask for SK_InsertSubvector (PR #85408)

[Alexey Bataev via llvm-commits]

================
@@ -7727,9 +7730,13 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
         }
         if (NeedInsertSubvectorAnalysis) {
           // Add the cost for the subvectors insert.
-          for (int I = VF, E = VL.size(); I < E; I += VF)
+          SmallVector<int> ShuffleMask(VL.size());
+          for (int I = VF, E = VL.size(); I < E; I += VF) {
+            for (int i = 0; i < E; i++)
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alexey-bataev wrote:

```suggestion
            for (int Idx : seq<int>(0, E))
```


https://github.com/llvm/llvm-project/pull/85408