[llvm] [VPlan] Add doesGenerateSingleScalar and use where applicable. (NFCI) (PR #222447)

[Luke Lau via llvm-commits]

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lukel97 wrote:

Whilst we're here I don't think isSingleScalar checks its users at all. I think this should be 
```suggestion
/// Returns true if \p VPV is a single scalar, either because it generates a single scalar or because it produces the same value for all lanes. 
bool isSingleScalar(const VPValue *VPV);
```

https://github.com/llvm/llvm-project/pull/222447