[llvm] [SLP] NFC. Add assert for ShuffleCostEstimator and ShuffleInstructionBuilder usage. (PR #123708)

[Han-Kuan Chen via llvm-commits]

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@@ -15229,6 +15235,9 @@ ResTy BoUpSLP::processBuildVector(const TreeEntry *E, Type *ScalarTy,
       SmallVector<int> BVMask(GatheredScalars.size(), PoisonMaskElem);
       TryPackScalars(GatheredScalars, BVMask, /*IsRootPoison=*/true);
       Value *BV = ShuffleBuilder.gather(GatheredScalars, BVMask.size());
+      assert((BVMask.size() == E->Scalars.size() ||
+              BVMask.size() == E->ReuseShuffleIndices.size()) &&
+             "The mask is incompatible with the expected result.");
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HanKuanChen wrote:

`BVMask` is initialized with `GatheredScalars.size()` and may be changed in `TryPackScalars` with `ReuseMask.assign(VF, PoisonMaskElem);`. The VF here is from `TreeEntry::getVectorFactor`.
`TryPackScalars` will make `GatheredScalars` to VF size. So the `BVMask.size()` is either `E->Scalars.size()` or `ReuseShuffleIndices.size()`.

https://github.com/llvm/llvm-project/pull/123708