[C++] Fails to detect control flow influence of nested “if”

The `controls` predicate in `GuardCondition` fails to detect a control flow influence from a nested `if`. In the following example the influence from `condition` to `call()` is only revealed in the first `if`, but not in the second.

```cpp
#include

void call()
{
}

void my_fn(bool outer, bool condition)
{

if (condition) // detected
{
throw std::exception();
}

if (outer)
{
if (condition) // not detected
{
throw std::exception();
}
}

call();
}
```

Query I tried:

```ql
import cpp
import semmle.code.cpp.controlflow.IRGuards

from Variable v, VariableAccess va, GuardCondition cond, Call c, int line
where
c.getTarget().getName() = "call" and
va.getTarget() = v and
v.getName() = "condition" and
cond.getAChild*() = va and
cond.controls(c.getBasicBlock(), _) and
line = va.getLocation().getStartLine()
select v, va, cond, c, line
```

Output I received:

```
| v | va | cond | c | line |
+-----------+-----------+-----------+--------------+------+
| condition | condition | condition | call to call | 10 |
```

I expected to also see an influence from line 17, but none is being found.

CodeQL version: 2.19.3

Start by reproducing the supplied C++ example with the query importing semmle.code.cpp.controlflow.IRGuards, then read GuardCondition and its controls predicate. The work is complete when the nested condition access on line 17 is reported as influencing call(), alongside the first condition.

Written by the indexing model from the issue text.