Introduce gpredict subclass and use it for all gimple_predict_ accessors

Introduce gpredict subclass and use it for all gimple_predict_ accessors

gcc/ChangeLog.gimple-classes:
	* coretypes.h (struct gpredict): Add forward declaration.
	* doc/gimple.texi (Class hierarchy of GIMPLE statements): Add
	gpredict.
	* gimple-pretty-print.c (pp_gimple_stmt_1): Within case
	GIMPLE_PREDICT, add local "predict_stmt" via a checked cast and
	use it in place of "stmt".
	* gimple.c (gimple_build_predict): Strengthen return type and
	local "p" from gimple to gpredict *.
	* gimple.h (struct gpredict): New subclass of
	gimple_statement_base, adding invariant that code is
	GIMPLE_PREDICT.
	(is_a_helper <gpredict *>::test): New.
	(gimple_build_predict): Strengthen return type from gimple to
	gpredict *.
	(gimple_predict_predictor): Strengthen param from const_gimple to
	const gpredict *.
	(gimple_predict_outcome): Likewise.
	(gimple_predict_set_predictor): Strengthen param from gimple to
	gpredict *.
	(gimple_predict_set_outcome): Likewise.
	* predict.c (tree_bb_level_predictions): Convert check for
	GIMPLE_PREDICT to a dyn_cast, introducing local "predict_stmt" and
	using it in place of "stmt" for typesafety.