[SLP]Improve handling gathers/buildvectors with undefs.

If have just one non-undef scalar in the buildvector/gather node, we try
to put it to be the very first element, which is profitable in most
cases. Do the preliminary estimation, if this more profitable during
graph rotation and do same for all elements, including extractelements.

Differential Revision: https://reviews.llvm.org/D144689