2018-08-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/86816
	* tree-ssa-tail-merge.c (tail_merge_valueize): New function
	which checks for value availability before querying it.
	(gvn_uses_equal): Use it.
	(same_succ_hash): Likewise.
	(gimple_equal_p): Likewise.

	* g++.dg/torture/pr86816.C: New testcase.

From-SVN: r263257