REFL0009 fires when result is checked with 'is not null'

[manfred-brands]

GetXAnalyzer.IsNullCheck doesn't recognize is not null.

The first code fragment raises REFL0009:

var property = type.GetProperty(PropertyName);
if (suppressionsProperty is not null)

The second does not:

var property = type.GetProperty(PropertyName);
if (suppressionsProperty != null)

[JohanLarsson]

Thanks for the issue, the analyzer was written before not was a thing. We must add support