Add gsi_after_labels overload for gimple_seq

The following adds gsi_after_labels for gimple_seq so I do not have
to open-code it.  I took the liberty to remove the two #defines
wrapping gsi_start_1 and gsi_last_1 as we now have C++ references.

2022-02-01  Richard Biener  <rguenther@suse.de>

	* gimple-iterator.h (gsi_after_labels): Add overload for
	gimple_seq.
	(gsi_start_1): Rename to gsi_start and take a reference.
	(gsi_last_1): Likewise.
	* gimple-iterator.cc (gsi_for_stmt): Use gsi_start.
	* omp-low.cc (lower_rec_input_clauses): Likewise.
	(lower_omp_scan): Likewise.