[SLP]Improve/fix reodering for gather nodes with extractelements/undefs.

If the gather node is a mix of undefvalues and exractelement
instructions, need to take the ordering for such nodes into account too.
It allows to reorder some (sub)trees and remove some extra shuffles,
improving overall vectorization.
Also, outlined common functionality into a separate function.

Differential Revision: https://reviews.llvm.org/D115358