Fix ?? inference and precedence

[RblSb]

Closes #11197
Closes #11239

Btw, this case fails for both if and null coal usages:

var b:Null<B> = cast null;
var c:C = cast null;
var a = if (b != null) b else c; // C should be Null<B>
var a = b ?? c;

[Simn]

I'm still not sure about this, something seems wrong with this precedence change, but I don't know what. Maybe it's just another situation where being pragmatic is the right choice.

[RblSb]

Other langs stats: tc39/proposal-nullish-coalescing#26 (comment)

[Simn]

Well, if there's no consensus anyway, I agree we can do our own thing for sure!