tree-optimization/95839 - allow CTOR vectorization without loads

tree-optimization/95839 - allow CTOR vectorization without loads

This removes a premature check for enough datarefs in a basic-block
before we consider vectorizing it which leaves basic-blocks with
just vectorizable vector constructors unvectorized.  The check
is effectively done by the following check for store groups
which then also include constructors.

2020-06-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95839
	* tree-vect-slp.c (vect_slp_analyze_bb_1): Remove premature
	check on the number of datarefs.

	* gcc.dg/vect/bb-slp-pr95839.c: New testcase.