do not pick full-packet if it'd result in more operations See comment and <https://gitlab.com/libeigen/eigen/merge_requests/46#note_270622952>.
Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.
For more information go to http://eigen.tuxfamily.org/.
For pull request, bug reports, and feature requests, go to https://gitlab.com/libeigen/eigen.