This causes no change in behavior, since code was alreadying doing
a no-op in bmesh_edge_separate if the edge is a boundary.
But it tripped an assert, annoying in debug builds.
We want to leave assert in bmesh_edge_separate in case callers
expect there to be separate loops after this always.
So putting test in caller.
(Same worry about bmesh_urmv_loop? I checked callers and they
appear OK to me - they deal with the no-op return.)
10c93a582b