* predict.c (return_prediction): New function.
(apply_return_prediction): Likewise.
(tree_bb_level_predictions): Likewise.
(tree_estimate_probability): Add noreturn exit edges; call
bb_level_predictions; fix logic of return heuristics.
(predict_paths_leading_to): Rescuesce from old CVS version of
process_note_prediction function.
* predict.def (PRED_TREE_EARLY_RETURN): New predictor.