No warning for identity pattern-matching comparison

[SLaks]

Version Used: a2df650

var b = 1 is 1 || 1 is 2;

Expected Behavior:
Warning: "The given expression is always / never equal to the provided value", just like is would when used with types.

Actual Behavior:
No warnings

http://tryroslyn.azurewebsites.net/#b:master/f:r/K4Zwlgdg5gBAygTxAFwKYFsDcAoADsAIwBswBjGUogQxBBgGEYBvbGNmfYsmANwHswAExgBZABQBKZtgCQMnlQBOMAjAC8MAIwwwdbQB99WnXQBMOOao3bdOiMhiHjtwX0JFUOdjAC+2H0A=

[SLaks]

Also, 1 is int x should perhaps give a warning (1 is var x should not).

[jcouv]

FYI @gafter

[gafter]

I imagine this is more likely to happen in a "warning wave" in a subsequent release, but I'll see if we can get it in to C# 7.

[alrz]

Is this related to unreachable code detection (rel. #15489)?

int M()
{
  if(1 is 1) return 2;
}

Same code with == does not produce any error.

[gafter]

Fixed in #26035