Change the way we split stores in BB vectorization

Change the way we split stores in BB vectorization

The following fixes missed optimizations due to the strange way we
split stores in BB vectorization.  The solution is to split at
the failure boundary and not re-align that to the initial piece
chosen vector size.  Also re-analyze any larger matching rest.

2020-10-28  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_build_slp_instance): Split the store
	group at the failure boundary and also re-analyze a large enough
	matching rest.

	* gcc.dg/vect/bb-slp-68.c: New testcase.