(left_shift_overflows, same_sign, overflow_sum_sign): New macros.

(add_double, div_and_round_double, lshift_double, mul_double):
(neg_double, const_binop, fold): Check for signed integer overflow.
Propagate overflow flags from operands to result.
(const_binop, fold_convert): Use pedwarn for overflow warnings.
Say `constant expression', not `constant folding', for user's sake.

From-SVN: r2053