[yul] ICE when accessing a Yul data subobject: `Assembly object not found or does not contain code`

## Description

```
object "t" {
code {
datacopy(not(datasize("object2.object3.object4.datablock")), 0, 0)
}
object "object2" {
code{}
object "object3" {
code{}
object "object4" {
code{}
data "datablock" ""
}
}
}
}
```

throws

https://github.com/ethereum/solidity/blob/4a478f0fb2d8d4f7bc7fdacfeb20e955d5396ce8/libyul/Object.cpp#L110

However

```
object "t" {
code {
datacopy(not(datasize("datablock")), 0, 0)
}
data "datablock" "okay"
}
```

compiles fine. So it seems data identifier is a valid identifier for `datasize/offset` opcode. However, nested data identifiers are not? Perhaps this is under specified?

## Environment

- Compiler version: latest develop

## Steps to Reproduce

```
$ solc --strict-assembly test.yul
```