2017-08-03  Richard Biener  <rguenther@suse.de>

	* tree-ssa-reassoc.c (should_break_up_subtract): Also break
	up if the use is in USE - X.

	* gcc.dg/tree-ssa/reassoc-23.c: Adjust to fool early folding
	and CSE.

From-SVN: r250855