Support `if` pattern in "use null coalescing assignment" refactoring 

Currently "use null coalescing assignment" refactoring only triggers for patterns like 
```cs
x = x ?? y;
```

It'd be nice to expand the support to `if` patterns

```cs
if (x == null)
{
    x = y;
}

if (x is null)
{
    x = y;
}
```