2018-01-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/81082
	* fold-const.c (fold_plusminus_mult_expr): Do not perform the
	association if it requires casting to unsigned.
	* match.pd ((A * C) +- (B * C) -> (A+-B)): New patterns derived
	from fold_plusminus_mult_expr to catch important cases late when
	range info is available.

	* gcc.dg/vect/pr81082.c: New testcase.
	* gcc.dg/tree-ssa/loop-15.c: XFAIL the (int)((unsigned)n + -1U) * n + n
	simplification to n * n.

From-SVN: r257077