Optimize away unused virtual implementations on abstract classes

[MichalStrehovsky]

This change lets us optimize away virtual method bodies on abstract classes that are overriden in all derived classes.

E.g.

class Base
{
    public virtual void DoSomething() => CallExpensiveMethod();
}

class Derived : Base
{
    public override void DoSomething() => SomeOtherMethod();
}

We shouldn't need to generate Base.DoSomething above since nobody calls that.

The optimization reuses the tentative instance methods introduced last week - the first commit just extracts that into a reusable component. The second commit makes us consider references to virtual method on abstract classes tentative, until e.g. a derived class that uses the base implementation makes them not tentative anymore.

We would probably get more bang if we could this to non-abstract classes too, but it's pretty difficult to discern "constructed EEType that is only constructed because it's a base of an allocated type". The biggest complication is reflection.

[jkotas]

I would be fine with this. If we have the capability, we can also use it where possible - even it means just 0.6% for what you have measured.

[MichalStrehovsky]

I would be fine with this. If we have the capability, we can also use it where possible - even it means just 0.6% for what you have measured.

Sounds good! I'll add the comments and tests tomorrow. Also I broke multifile again so I gotta fix that (I kind of expected that).

[jkotas]

Thank you!

[MichalStrehovsky]

Who disabled "rebase and merge" and for what reason? Sigh. Merge commit it is.

[jkotas]

I was asked to disable it to match dotnet/runtime.

[MichalStrehovsky]

I was asked to disable it to match dotnet/runtime.

Sigh. I remember that issue in the runtime repo and it didn't make sense to me at that time and it doesn't make sense to me today after re-reading it either. Yay for consistency.