[llvm] [LV] Improve materialisation of VFxUF for scalable vectors (PR #216708)

[David Sherwood via llvm-commits]

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david-arm wrote:

And the VF is still materialised as

```
  VF.replaceAllUsesWith(RuntimeVF);
```

which looks correct.

https://github.com/llvm/llvm-project/pull/216708