[VPlan] Relax over-aggressive assertion in VPTransformState::get().

There are cases where a vector value has some users that demand the
the single scalar value only (NeedsScalar), while other users demand the
vector value (see attached test cases). In those cases, the NeedsScalar
users should only demand the first lane.

Fixes https://github.com/llvm/llvm-project/issues/91883.