tree-optimization/95219 - improve IV selection for induction

This improves code generation with SSE2 for the testcase by
making sure to only generate a single IV when the group size
is a multiple of the vector size.  It also adjusts the testcase
which was passing before.

2020-05-20  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95219
	* tree-vect-loop.c (vectorizable_induction): Reduce
	group_size before computing the number of required IVs.

	* gcc.dg/vect/costmodel/x86_64/costmodel-pr30843.c: Adjust.