[llvm] [SLP] De-duplicate costing of perfect-diamond gather spills (PR #220569)

[Alexey Bataev via llvm-commits]

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@@ -19040,6 +19045,77 @@ InstructionCost BoUpSLP::getSpillCost() {
     }
     return nullptr;
   };
+  auto IsCoveredByMatchingVectorEntry = [&](const TreeEntry *Gather,
+                                            const Loop *SpillLoop) -> bool {
+    assert(Gather->isGather() && "Expected a Gather Entry!");
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alexey-bataev wrote:

```suggestion
    assert(Gather->isGather() && "Expected a gather/buildvector entry.");
```

https://github.com/llvm/llvm-project/pull/220569