tree-optimization/98526 - fix vectorizer reduction cost

tree-optimization/98526 - fix vectorizer reduction cost

This fixes a double-counting in the reduction cost when vectorizing
the reduction through the regular vectorizable_* functions.

2021-01-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98526
	* tree-vect-loop.c (vect_model_reduction_cost): Remove costing
	of the actual reduction op for the regular case.
	(vectorizable_reduction): Cost the stmts
	vect_transform_reduction produces here.

(cherry picked from commit 04bff1bbfc11a974342c0eb0c0d65d902e36e82e)