[SLP]Try to vectorize scalars, being vectorized already, but does not need to be scheduled.

If the scalar does not need to be scheduled and it was vectorized
already in one of the vector nodes, we still can try to vectorize it in
another node. Just does not need account its cost in the scalar total
cost, as it will be handled in the main vectorized node.

Differential Revision: https://reviews.llvm.org/D159205