Make all gimple_omp_return_ accessors typesafe

Make all gimple_omp_return_ accessors typesafe

gcc/ChangeLog.gimple-classes:
	* gimple.h (struct gimple_statement_omp_return): Rename to...
	(struct gomp_return): ...this.
	(is_a_helper <gimple_statement_omp_return *>::test): Rename to...
	(is_a_helper <gomp_return *>::test): ...this.
	(is_a_helper <const gimple_statement_omp_return *>::test): Rename to...
	(is_a_helper <const gomp_return *>::test): ...this.
	(gimple_build_omp_return): Strengthen return type from gimple to
	gomp_return *.
	(gimple_omp_return_set_nowait): Likewise for param.
	(gimple_omp_return_set_lhs): Likewise.
	(gimple_omp_return_lhs_ptr): Likewise.
	(gimple_omp_return_nowait_p): Strengthen param from const_gimple
	to const gomp_return *.
	(gimple_omp_return_lhs): Likewise.

	* gimple-pretty-print.c (dump_gimple_omp_return): Strengthen param
	from gimple to gomp_return *.
	(pp_gimple_stmt_1): Add checked cast.
	* gimple-walk.c (walk_gimple_op): Likewise.
	* gimple.c (gimple_build_omp_return): Strengthen return type and
	local "p" from gimple to gomp_return *.
	* omp-low.c (remove_exit_barrier): Replace check against
	GIMPLE_OMP_RETURN with a dyn_cast, introducing local
	"omp_return_stmt" and using it in place of "stmt".
	(expand_omp_for_generic): Introduce local "omp_return_stmt" via a
	checked cast, using it in place of "gsi_stmt (gsi)".
	(expand_omp_for_static_nochunk): Likewise.
	(expand_omp_for_static_chunk): Likewise.
	(expand_omp_sections): Introduce local "omp_return_stmt" via a
	checked cast, using it in place of "gsi_stmt (si)".
	(expand_omp_single): Likewise.
	(maybe_add_implicit_barrier_cancel): Strengthen local "omp_return"
	from gimple to gomp_return * via a checked cast.