fold-const.c (fold_negate_expr): Don't fold INTEGER_CST if that overflows when SANITIZE_SI_OVERFLOW is on.

	* fold-const.c (fold_negate_expr): Don't fold INTEGER_CST if
	that overflows when SANITIZE_SI_OVERFLOW is on.  Guard -(-A)
	folding with TYPE_OVERFLOW_SANITIZED.

	* c-c++-common/ubsan/overflow-negate-3.c: New test.

From-SVN: r217556