middle-end/94465 - handle released SSA names in array_ref_low_bound

middle-end/94465 - handle released SSA names in array_ref_low_bound

array_ref_low_bound is used in dumping ARRAY_REFs which in turn
is called when basic blocks are deleted.  cleanup_control_flow_pre
consciously decides to remove unreachable basic-blocks in arbitrary
order so the following makes array_ref_low_bound forgiving in the
case the SSA name with the index definition has been released
already.

2020-04-03  Richard Biener  <rguenther@suse.de>

	PR middle-end/94465
	* tree.c (array_ref_low_bound): Deal with released SSA names
	in index position.