iredges: get rid of edge_change_cnt function

The operation that function did was trivial, not abstracting it avoids repeated
get_irn_edge_info() calls in some cases.
fix incorrect edges_notify_edge in remove_irn_n
iredges: add some edge count sanity checking asserts
simplify remove_End_keepalive()
use remove_End_n() instead of remove_End_keepalive() where possible
Cleanup using C99.
Improved readability.
Removed superfluous check.