tree-optimization/97706 - handle PHIs in pattern recog mask precison

This adds handling of PHIs to mask precision compute which is
eventually needed to detect a bool pattern when the def chain
contains such a PHI node.

2020-11-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97706
	* tree-vect-patterns.c (possible_vector_mask_operation_p):
	PHIs are possible mask operations.
	(vect_determine_mask_precision): Handle PHIs.
	(vect_determine_precisions): Walk PHIs in BB analysis.

	* gcc.dg/vect/bb-slp-pr97706.c: New testcase.