tree-optimization/102696 - fix SLP discovery for failed BIT_FIELD_REF

This fixes a forgotten adjustment of matches[] when we fail SLP
discovery.

2021-10-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102696
	* tree-vect-slp.c (vect_build_slp_tree_2): Properly mark
	the tree fatally failed when we reject a BIT_FIELD_REF.

	* g++.dg/vect/pr102696.cc: New testcase.