Allow more BB vectorization

Allow more BB vectorization

The following allows more BB vectorization by generally building leafs
from scalars rather than giving up.  Note this is only a first step
towards this and as can be seen with the exception for node splitting
it is generally hard to get this heuristic sound.  I've added variants
of the bb-slp-48.c testcase to make sure we still try permuting for
example.

2020-09-15  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_build_slp_tree_2): Also consider
	building an operand from scalars when building it did not
	fail fatally but avoid messing with the upcall splitting
	of groups.

	* gcc.dg/vect/bb-slp-48.c: New testcase.
	* gcc.dg/vect/bb-slp-7.c: Adjust.