Refactor SLP permute opt propagation

Refactor SLP permute opt propagation

This rewrites the SLP permute opt propagation to elide the visited
bit for an incoming permute of -1 as well as allowing the initial
propagation to take more than one iteration before starting on
materialization.  As we still lack propagation in the reverse
direction I've added gcc.dg/vect/bb-slp-71.c and a stopgap to
restrict "any" permute handling to the supported cases.

2021-06-29  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (slpg_vertex::visited): Remove.
	(vect_slp_perms_eq): Handle -1 permutes.
	(vect_optimize_slp): Rewrite permute propagation.

	* gcc.dg/vect/pr67790.c: Un-XFAIL.
	* gcc.dg/vect/bb-slp-71.c: New testcase.