how the read-write analysis works for loops

### Describe the issue:

When slither generates IFLOOP node, it has every read and write from its body.

### Code example to reproduce the issue:

```solidity
contract A {
function foo() public pure {
int i = 4;
while (i < 4) {
i++;
}
}
```
```python
>>> from slither import Slither
>>> foo = Slither('a.sol').contracts[0].functions[0]
>>> print(foo.nodes[3])
IF_LOOP i < 4
>>> print(foo.nodes[3].variables_written[0])
i
```

### Version:

0.9.3

### Relevant log output:

_No response_