Remove unnecessary __hx_cast calls

[waneck]

Performance-related: Specially on AOT, __hx_cast<> calls are expensive, and they get called even when it isn't needed. Maybe a cleanup on unnecessary TCasts would be a good way to solve this.

[dogles]

Is there a way to handle the general case (no conversion needed) without generic function call overhead? I'm thinking something like (not valid C#):

object __hx_cast(System.Type toType) { return (typeof(T) != toType) ? this.MyClass_cast<toType>() : this; }