Decide on LOOP_VINFO_USING_SELECT_VL_P after determining partial vectors

Decide on LOOP_VINFO_USING_SELECT_VL_P after determining partial vectors

The following makes us decide on partial vectors first so we can
use LOOP_VINFO_USING_PARTIAL_VECTORS_P to decide on a decrementing IV
and LOOP_VINFO_USING_SELECT_VL_P as followup.

	* tree-vect-loop.cc (vect_determine_partial_vectors_and_peeling):
	Remove resetting of LOOP_VINFO_USING_SELECT_VL_P.
	(vect_analyze_loop_2): Decide on partial vectors before
	deciding on decrementing IV or .SELECT_VL usage.