// If not, return OXXX. In this case, the string return parameter may
// hold a reason why. In all other cases, it'll be the empty string.
// srcConstant indicates whether the value of type src is a constant.
func convertOp(srcConstant bool, src, dst *types.Type) (ir.Op, string) {
	if src == dst {
		return ir.OCONVNOP, ""
	}
	if src == nil || dst == nil {
		return ir.OXXX, ""
	}