tree-optimization/101242 - fix reverse graph entry detection

tree-optimization/101242 - fix reverse graph entry detection

This avoids detecting random unrelated nodes as possible entries
to not backwards reachable regions of the SLP graph.  Instead
explicitely add the problematic nodes.

This temporary XFAILs gcc.dg/vect/pr67790.c until I get the
permute propagation adjusted to when it needs more than one
optimistic iteration.

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

	PR tree-optimization/101242
	* tree-vect-slp.c (vect_slp_build_vertices): Force-add
	PHIs with not represented initial values as leafs.

	* gcc.dg/vect/bb-slp-pr101242.c: New testcase.
	* gcc.dg/vect/pr67790.c: XFAIL scan for zero VEC_PERM_EXPR.