More operators inside `is(...)` expressions

[Steven Schveighoffer]

On 8/23/20 5:52 PM, Dennis wrote:
> On Sunday, 23 August 2020 at 21:20:21 UTC, Steven Schveighoffer wrote:
>> What is the cost here? Just use the same AST node for the is 
>> expression, and wrap it in a `not` AST node.
> 
> What would `is(int != T, T)` mean? To me it sounds like T should be 
> bound to an arbitrary type that's not int, but when you lower it to 
> `!is(int == T, T)` it returns false.

BTW, what I would propose is that the following case be added as an is 
Expression:

is(Type != TypeSpecialization)

As syntax sugar for:

!is(Type == TypeSpecialization)

And no other forms using != are valid.

-Steve