make swap argument of vect_get_and_check_slp_defs readonly

make swap argument of vect_get_and_check_slp_defs readonly

Since some time we're only using this argument to communicate from
vect_build_slp_tree_1 to vect_get_and_check_slp_defs.  This makes
the direction of information flow clear.

2020-09-15  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_get_and_check_slp_defs): Make swap
	argument by-value and do not change it.
	(vect_build_slp_tree_2): Adjust, set swap to NULL after last
	use.