Allow single-lane SLP in-order reductions

Allow single-lane SLP in-order reductions

The single-lane case isn't different from non-SLP, no re-association
implied.  But the transform stage cannot handle a conditional reduction
op which isn't checked during analysis - this makes it work, exercised
with a single-lane non-reduction-chain by gcc.target/i386/pr112464.c

	* tree-vect-loop.cc (vectorizable_reduction): Allow
	single-lane SLP in-order reductions.
	(vectorize_fold_left_reduction): Handle SLP reduction with
	conditional reduction op.