Unexpected/misleading IDE diagnostics for a user defined assignment operator usage

```
class Program
{
static void Main()
{
var c1 = new C1();
c1++; // IDE0059 Unnecessary assignment of a value to 'c1'

var c2 = new C1();
c2 -= 1; // IDE0059 Unnecessary assignment of a value to 'c2'
}
}

class C1
{
public void operator ++()
{
}

public void operator -=(int i)
{
}
}
```

Start by reproducing the C# snippet and investigating the IDE0059 diagnostic for the user-defined ++ and -= operators. Trace the analyzer entry point and existing diagnostic tests, then add a regression test showing that these operator usages no longer receive the misleading unnecessary-assignment warning.

Written by the indexing model from the issue text.