Finding which loop should share its vertex with which others is not easy
with regular Mesh data (mostly due to lack of advanced topology info, as
opposed with BMesh case).
Custom loop normals computing already does that - and can return 'loop
normal spaces', which among other things contain definitions of 'smooth
fans' of loops around vertices.
Using those makes it easy to find vertices (and then edges) that needs
splitting.
This commit also adds support of non-autosmooth meshes, where we want to
split out flat faces from smooth ones.
ae1c1cd8c0