Fix S1125 FN: recognize "is" keyword with constant pattern

[rangp]

Description

Starting with c# 7.0 the is keyword can be used with a constant pattern as an alternative to == or .Equals(). This is not yet covered by the S1125 rule

Repro steps

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

if (x is true)
{
   // no S1125 warning
}

Expected behavior

Both of the above statements should get the S1125 warning since they're equivalent

Actual behavior

Only the first statement gets the warning

Known workarounds

None

[andrei-epure-sonarsource]

thanks @rangp , you're right!

[pavel-mikula-sonarsource]

..when the tested expression is bool

For nullable value type bool?, the is true or is false does also a null check.