tree-optimization/39821 - fix cost classification for widening arith

tree-optimization/39821 - fix cost classification for widening arith

This adjusts the vectorizer to cost vector_stmt for widening
arithmetic instead of vec_promote_demote in the line of telling
the target that stmt_info->stmt is the meaningful piece we cost.

2021-07-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/39821
	* tree-vect-stmts.c (vect_model_promotion_demotion_cost): Use
	vector_stmt for widening arithmetic.
	(vectorizable_conversion): Adjust.