Refactor BB splitting of DRs for SLP group analysis

Refactor BB splitting of DRs for SLP group analysis

This uses the group_id computed to ensure DRs in different BBs do
not get merged into a DR group.  To achieve this we seed the
group from the BB index when group_ids are not computed and we
make sure to bump the group_id when advancing to the next BB for
BB SLP analysis.

This paves the way for relaxing the grouping for BB vectorization
by adjusting its group_id computation.

2021-08-20  Richard Biener  <rguenther@suse.de>

	* tree-vect-data-refs.c (dr_group_sort_cmp): Do not compare
	BBs.
	(vect_analyze_data_ref_accesses): Likewise.  Assign the BB
	index as group_id when dataref_groups were not computed.
	* tree-vect-slp.c (vect_slp_bbs): Bump current_group when
	we advace to the next BB.

(cherry picked from commit 37744f8260857005c8409c9e2e633a05c768a7dd)