[llvm] [VPlan] Introduce vputils::onlyScalarValuesUsed (NFC) (PR #153577)

[Florian Hahn via llvm-commits]

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@@ -25,6 +25,9 @@ bool onlyFirstLaneUsed(const VPValue *Def);
 /// Returns true if only the first part of \p Def is used.
 bool onlyFirstPartUsed(const VPValue *Def);
 
+/// Returns true if only scalar values of \p Def is used by all users.
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fhahn wrote:

```suggestion
/// Returns true if only scalar values of \p Def are used by all users.
```

https://github.com/llvm/llvm-project/pull/153577