addSaveErrRetIndex

    fn addSaveErrRetIndex(
        gz: *GenZir,
        cond: union(enum) {
            always: void,
            if_of_error_type: Zir.Inst.Ref,
        },
    ) !Zir.Inst.Index {
        return gz.addAsIndex(.{
            .tag = .save_err_ret_index,
            .data = .{ .save_err_ret_index = .{
                .operand = switch (cond) {
                    .if_of_error_type => |x| x,
                    else => .none,
                },
            } },
        });
    }