tree-optimization/97753 - fix SLP induction vect

This fixes updating of the step vectors when filling up to group_size.

2020-11-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97753
	* tree-vect-loop.c (vectorizable_induction): Fill vec_steps
	when CSEing inside the group.

	* gcc.dg/vect/pr97753.c: New testcase.