[SLP]Improve gathering of the scalars used in the graph.

Currently we emit gathers for scalars being vectorized in the tree as
a pair of extractelement/insertelement instructions. Instead we can try
to find all required vectors and emit shuffle vector instructions
directly, improving the code and reducing compile time.

Part of non-power-of-2 vectorization.

Differential Revision: https://reviews.llvm.org/D110978