[VPlan] Reuse introduces-broadcast logic in narrowToSingleScalars (#174444)

narrowToSingleScalarRecipes' operands check is a bit too restrictive by
permitting a single user. Factor out and reuse the existing
introduces-broadcast logic to improve results.

GitOrigin-RevId: ec0b22ff472368e11de25e0f9d38527c937500f2
4 files changed