Remove LOOP_VINFO_SLP_UNROLLING_FACTOR

Remove LOOP_VINFO_SLP_UNROLLING_FACTOR

The following removes LOOP_VINFO_SLP_UNROLLING_FACTOR in favor of
using LOOP_VINFO_VECT_FACTOR directly now that there's no difference
between the two possible.

	* tree-vectorizer.h (_loop_vec_info::slp_unrolling_factor): Remove.
	(LOOP_VINFO_SLP_UNROLLING_FACTOR): Likewise.
	* tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Adjust.
	(vect_analyze_loop_2): Likewise.
	* tree-vect-slp.cc (vect_make_slp_decision): Set
	LOOP_VINFO_VECT_FACTOR directly.