add vect_get_slp_vect_def

add vect_get_slp_vect_def

This adds vect_get_slp_vect_def to get at a SLP nodes vectorized def,
abstracting away the details.  It also fixes one stray failure to
use SLP_TREE_REPRESENTATIVE.

2020-05-04  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vect_get_slp_vect_def): Declare.
	* tree-vect-loop.c (vect_create_epilog_for_reduction): Use it.
	* tree-vect-stmts.c (vect_transform_stmt): Likewise.
	(vect_is_simple_use): Use SLP_TREE_REPRESENTATIVE.
	* tree-vect-slp.c (vect_get_slp_vect_defs): Fold into single
	use ...
	(vect_get_slp_defs): ... here.
	(vect_get_slp_vect_def): New function.