[SLP]Fix PR74607: Fix dependency between buildvector nodes with user

nodes, having same last instruction.

If the user nodes has the same last-instruction, used as insert points
for the buildvector nodes, finding the proper dependency is crucial.
  Before, it depended on the indices of the buildvectors themselves but
  looks like it should depend on indices of the user nodes, because it
  identifies the vectorization order and, thus, properly aligns
  buildvector nodes in terms of def-use chain.