Support single def-use cycle optimization for SLP reduction vectorization

Support single def-use cycle optimization for SLP reduction vectorization

We can at least mimic single def-use cycle optimization when doing
single-lane SLP reductions and that's required to avoid regressing
compared to non-SLP.

	* tree-vect-loop.cc (vectorizable_reduction): Allow
	single-def-use cycles with SLP.
	(vect_transform_reduction): Handle SLP single def-use cycles.
	(vect_transform_cycle_phi): Likewise.

	* gcc.dg/vect/slp-reduc-12.c: New testcase.