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

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

Similar to https://github.com/llvm/llvm-project/pull/196181 actually -- that patch is strictly more powerful, because it also make a more fine-grained distinction taking vector-to-scalar and scalar-to-vector into account?

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