tree-optimization/101028 - fix endless SLP reassoc discovery

This fixes a missing clearing of mismatched lanes from the
fatal fail path in SLP reassoc discovery in the most conservative
way.

2021-06-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101028
	* tree-vect-slp.c (vect_build_slp_tree_2): When SLP
	reassoc discovery fails fatally, mark appropriate lanes
	in matches[] so.

	* gcc.dg/pr101028.c: New testcase.