avm2: Make optimizer lookup callee return type

[Aaron1011]

This provides us with more optimization opportunities. Since we optimize methods one at a time, we need to look up the return type in the domain rather than relying on the resolved return type in the target BytecodeMethod

[Lord-McSweeney]

I don't think this works; for example

class A {
    public function someMethod():MyType;
}
class B extends A {
    public override function someMethod():OtherType;
}

If a type the optimizer knows is of type A's someMethod is called, the optimizer assumes the return type is of type MyType but it could also be of type OtherType. We need to verify that overriding methods have the same return type as their supermethods.

[adrian17]

To be clear: it is correct and works on real SWFs, we just don't validate them as strictly as FP does (so it can break on invalid SWFs)?

[Lord-McSweeney]

The classes

class A {
    public function someMethod():MyType;
}
class B extends A {
    public override function someMethod():OtherType;
}

throws a VerifyError in FP. I'm working on getting it to do so in Ruffle too.