tree-optimization/97706 - part one, refactor vect_determine_mask_precision

This computes vect_determine_mask_precision in a RPO forward walk
rather than in a backward walk and using a worklist.  It will make
fixing PR97706 easier but for bisecting I wanted it to be separate.

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

	PR tree-optimization/97706
	* tree-vect-patterns.c (vect_determine_mask_precision):
	Remove worklist operation.
	(vect_determine_stmt_precisions): Do not call
	vect_determine_mask_precision here.
	(vect_determine_precisions): Compute mask precision
	in a forward walk.