[llvm] [VPlan] Implement interleaving as VPlan-to-VPlan transform. (PR #95842)

[via llvm-commits]

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@@ -3507,6 +3538,10 @@ class VPlan {
   bool hasScalarVFOnly() const { return VFs.size() == 1 && VFs[0].isScalar(); }
 
   bool hasUF(unsigned UF) const { return UFs.empty() || UFs.contains(UF); }
+  unsigned getUF() const {
+    assert(UFs.size() == 1);
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ayalz wrote:

```suggestion
    assert(UFs.size() == 1 && "A single UF expected");
```

https://github.com/llvm/llvm-project/pull/95842