Fix S1125 FN: recognize C#9 "is not" constant pattern

[cristian-ambrosini-sonarsource]

Following #2619, S1125 should cover C#9 feature NotPattern "is not".

var x = false;
if (x is true)
{
   // S1125 warning
}

if (x is not true)
{
   // no S1125 warning
}