tree-optimization/97428 - split SLP groups for loop vectorization

tree-optimization/97428 - split SLP groups for loop vectorization

This enables SLP store group splitting also for loop vectorization.
For the existing testcase gcc.dg/vect/vect-complex-5.c this then
generates much better code, likewise for the PR97428 testcase.

Both of those have a splitting opportunity splitting the group
into two equal (vector-sized) halves, still the patch enables
quite arbitrary splitting since generally the interleaving scheme
results in quite awkward code for even small groups.  If any
problems surface with this it's easy to restrict the splitting
to known-good cases.

2020-10-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97428
	* tree-vect-slp.c (vect_analyze_slp_instance): Split store
	groups also for loop vectorization.

	* gcc.dg/vect/vect-complex-5.c: Expect to SLP.
	* gcc.dg/vect/pr97428.c: Likewise.