create reference variable for offset and length in Yul

### Describe the issue:

`write-after-write` detector falsely reacts on assembly work with calldata:

### Code example to reproduce the issue:

```
function foo(bytes calldata cd) private pure returns (bytes calldata signature)
{
assembly {
let ptr := add(cd.offset, calldataload(cd.offset))
signature.offset := add(ptr, 0x20)
signature.length := calldataload(ptr)
}
}
```

### Version:

^0.9 - tool was run on other machine

### Relevant log output:

_No response_

Reproduce the false positive with the provided foo(bytes calldata cd) Yul example and the available Slither version. Inspect the write-after-write detector's handling of calldata offset and length assignments; done means this example no longer reports a false positive and regression coverage verifies that behavior.

Written by the indexing model from the issue text.