[SLP]Unify vectorization of PHI and store nodes with improved tiny tree vectorization.

Vectorization of PHIs and stores very similar, it might be beneficial to
try to revectorize stores (like PHIs) if the total number of stores with
the same/alternate opcode is less than the vector size but number of
stores with the same type is larger than the vector size.

Differential Revision: https://reviews.llvm.org/D109831