addRestoreErrRetIndex

    fn addRestoreErrRetIndex(
        gz: *GenZir,
        bt: BranchTarget,
        cond: union(enum) {
            always: void,
            if_non_error: Zir.Inst.Ref,
        },
        src_node: Ast.Node.Index,
    ) !Zir.Inst.Index {
        switch (cond) {
            .always => return gz.addAsIndex(.{
                .tag = .restore_err_ret_index_unconditional,
                .data = .{ .un_node = .{
                    .operand = switch (bt) {
                        .ret => .none,
                        .block => |b| b.toRef(),
                    },
                    .src_node = gz.nodeIndexToRelative(src_node),
                } },
            }),
            .if_non_error => |operand| switch (bt) {
                .ret => return gz.addAsIndex(.{
                    .tag = .restore_err_ret_index_fn_entry,
                    .data = .{ .un_node = .{
                        .operand = operand,
                        .src_node = gz.nodeIndexToRelative(src_node),
                    } },
                }),
                .block => |block| return (try gz.addExtendedPayload(
                    .restore_err_ret_index,
                    Zir.Inst.RestoreErrRetIndex{
                        .src_node = gz.nodeIndexToRelative(src_node),
                        .block = block.toRef(),
                        .operand = operand,
                    },
                )).toIndex().?,
            },
        }
    }