tree-optimization/97746 - fix order of mask precision computes

This fixes the order of walking PHIs and stmts for BB mask
precision compute.

2020-11-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97746
	* tree-vect-patterns.c (vect_determine_precisions): First walk PHIs.

	* gcc.dg/vect/bb-slp-pr97746.c: New testcase.