See if we can optimize blocks under null checks

[MichalStrehovsky]

In the below program we should know that something typed as ITestOutput won't ever be non-null. We should be able to optimize out the null check branch in Foo.

using System;
using System.Runtime.CompilerServices;

class Program
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Foo(ITestOutput? to = null)
    {
        to?.Log("hello");
    }

    static void Main() => Foo(null);
}

interface ITestOutput
{
    void Log(string s);
}

Cc @nagya @jevansaks @EgorBo

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