tree-optimization/97334 - improve BB SLP discovery

tree-optimization/97334 - improve BB SLP discovery

We're running into a multiplication with one unvectorizable
operand we expect to build from scalars but SLP discovery
fatally fails the build of both since one stmt is commutated:

  _60 = _58 * _59;
  _63 = _59 * _62;
  _66 = _59 * _65;
...

where _59 is the "bad" operand.  The following patch makes the
case work where the first stmt has a good operand by not fatally
failing the SLP build for the operand but communicating upwards
how to commutate.

2020-10-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97334
	* tree-vect-slp.c (vect_build_slp_tree_1): Do not fatally
	fail lanes other than zero when BB vectorizing.

	* gcc.dg/vect/bb-slp-pr65935.c: Amend.