Want query to detect mutual assignment

**Description of the issue**

Here is a minimal test case showing mutual assignment where we do `b = a;` followed by `a = b;`:

```
int main(void) {
int a = 0;
int b = a;
int c = 1;
a = b;
return a*b*c;
}
```

The statement `a = b;` is redundant. Unless the variables are marked volatile, there should be no legitimate reason for code to do this. We just found an instance of this in OpenZFS:

https://github.com/openzfs/zfs/pull/14565/files#r1124115812

A quick search revealed that PVS Studio has a check for this:

https://pvs-studio.com/en/docs/warnings/v587/