Remove multi-line macro (FOR_EACH_PRED) in backgraph

Remove multi-line macro (FOR_EACH_PRED) in backgraph
(code refactoring)

This change simplifies stepping thru backgraph code in gdb.

* backgraph.c (FOR_EACH_PRED): Remove macro.
* backgraph.c (add_edge): Rename old_back_ptr local variable to pred.
* backgraph.c (add_edge, backwards_height, update_max_height): Expand
FOR_EACH_PRED macro.
* backgraph.c (backwards_height): Rename back_ptr local variable to
pred.
* backgraph.c (backwards_height, update_max_height): Rename q local
variable to pred.
* backgraph.c (update_max_height): Rename q_height local variable to
this_height.