[llvm] [VPlan] Run narrowInterleaveGroups during general VPlan optimizations. (PR #149706)

[via llvm-commits]

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@@ -4152,6 +4152,9 @@ class VPlan {
   /// Represents the vectorization factor of the loop.
   VPValue VF;
 
+  /// Represents the symbolic unroll factor of the loop.
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ayalz wrote:

```suggestion
  /// Represents the unroll factor of the loop.
```
VF and VFxUF are also "symbolic", when VF is fixed.
Worth documenting here that they must not be used after materializing?

https://github.com/llvm/llvm-project/pull/149706