return edge in make_eh_edges

The need to initialize edge probabilities has made make_eh_edges
undesirably hard to use.  I suppose we don't want make_eh_edges to
initialize the probability of the newly-added edge itself, so that the
caller takes care of it, but identifying the added edge in need of
adjustments is inefficient and cumbersome.  Change make_eh_edges so
that it returns the added edge.


for  gcc/ChangeLog

	* tree-eh.cc (make_eh_edges): Return the new edge.
	* tree-eh.h (make_eh_edges): Likewise.