Check whether blocking is worthwhile for custom types

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Link to original bugzilla bug (#1759)
Version: 3.4 (development)

Description

Related to Bug #1758 (and https://stackoverflow.com/questions/58366939/eigen3-boost-mpfr-segfault-when-alignment-is-disabled), it may actually not worth to use blocking algorithms for custom types, especially if sub-optimal cache access is less expensive than copying.

This not only applies to GEMM/GEMV products but also, e.g., triangular solvers.

Blocking

#1608