[SLP]Improve/fix reordering of the gathered graph nodes.

Gathered loads/extractelements/extractvalue instructions should be
checked if they can represent a vector reordering node too and their
order should ve taken into account for better graph reordering analysis/
Also, if the gather node has reused scalars, they must be reordered
instead of the scalars themselves.

Differential Revision: https://reviews.llvm.org/D112454