gcov: Add gen_counter_update()

Move the counter update to the new gen_counter_update() helper function.  Use
it in gimple_gen_edge_profiler() and gimple_gen_time_profiler().  The resulting
gimple instructions should be identical with the exception of the removed
unshare_expr() call.  The unshare_expr() call was used in
gimple_gen_edge_profiler().

gcc/ChangeLog:

	* tree-profile.cc (gen_assign_counter_update): New.
	(gen_counter_update): Likewise.
	(gimple_gen_edge_profiler): Use gen_counter_update().
	(gimple_gen_time_profiler): Likewise.