flow-analysis: increment decrement operators

I believe the following code is not producing the expected result or diagnostics.

```c
#pragma safety enable

void f(char* p)
{
    while (p)
    {
      if (*p != '\0') {}
      *p = 0;
      p++; //p++ must change the state to unkown
    }
}
```
Cake 0.13.27
main.c:7:11: warning 28: value is always zero
 7 |      if (*p != '\0') {}
   |          ~~             