* fold-const.c (fold_binary_loc): Don't fold if the result
	is undefined.
	* match.pd (A + (-B) -> A - B, A - (-B) -> A + B,
	-(-A) -> A): Likewise.

	* c-c++-common/ubsan/overflow-sub-4.c: New test.
	* c-c++-common/ubsan/overflow-sub-2.c: Adjust dg-output.
	* c-c++-common/ubsan/overflow-int128.c: Likewise.

From-SVN: r217427