PR rtl-optimization/78546
	* simplify-rtx.c (neg_const_int): When negating most negative
	number in mode wider than HOST_BITS_PER_WIDE_INT, use
	simplify_const_unary_operation to produce CONST_DOUBLE or
	CONST_WIDE_INT.
	(simplify_plus_minus): Hanlde the case where neg_const_int
	doesn't return a CONST_INT.

	* gcc.dg/torture/pr78546-1.c: New test.
	* gcc.dg/torture/pr78546-2.c: New test.

From-SVN: r242929