[SLP]Find reused scalars in buildvector sequences, if any.

Patch generalizes analysis of scalars. The main part is outlined into
lambda, which can be used to find reused inserted scalars and emit
shuffle for them instead of multiple insertelement instructions, if the
permutation is found alreadyi. I.e. some scalars are transformed by the
permutation of previously vectorized nodes, and some are inserted
directly.

Reworked part of D110978

Differential Revision: https://reviews.llvm.org/D146564