Rely on PGO for isinst/castclass

[EgorBo]

Follow up to #65460 (and d-o)

Example (with DOTNET_JitCastProfiling=1):

using System.Runtime.CompilerServices;
using System.Threading;

public interface IClass {}
public class ClassA : IClass {}
public class ClassB : ClassA {}


public class Program
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    static ClassA CastToClassA(object o) => (ClassA)o; // currently we always emit a fast path for ClassA here
                                                       // but in my case o is always ClassB (PGO helps)

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsClassA(object o) => o is ClassA;     // we don't expand it without PGO

    [MethodImpl(MethodImplOptions.NoInlining)]
    static IClass CastToIClass(object o) => (IClass)o; // we don't expand it without PGO

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsIClass(object o) => o is IClass;     // we don't expand it without PGO


    public static void Main()
    {
        // promote methods to tier1
        var b = new ClassB();
        for (int i = 0; i < 100; i++)
        {
            CastToClassA(b);
            IsClassA(b);
            CastToIClass(b);
            IsIClass(b);
            Thread.Sleep(16);
        }
    }
}

Codegen diff: https://www.diffchecker.com/RFblv9RB

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Follow up to #65460

Example (with DOTNET_JitCastProfiling=1):

using System.Runtime.CompilerServices;
using System.Threading;

public interface IClass {}
public class ClassA : IClass {}
public class ClassB : ClassA {}


public class Program
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    static ClassA CastToClassA(object o) => (ClassA)o;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsClassA(object o) => o is ClassA;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static IClass CastToIClass(object o) => (IClass)o;

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool IsIClass(object o) => o is IClass;


    public static void Main()
    {
        // promote methods to tier1
        var b = new ClassB();
        for (int i = 0; i < 100; i++)
        {
            CastToClassA(b);
            IsClassA(b);
            CastToIClass(b);
            IsIClass(b);
            Thread.Sleep(16);
        }
    }
}

Codegen diff: https://www.diffchecker.com/RFblv9RB

Author:	EgorBo
Assignees:	EgorBo
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

@AndyAyersMS @jakobbotsch PTAL

This PR consumes profile data from mibc for casts/isinst. I guarded it with DOTNET_JitConsumeProfileForCasts just to wait till dotnet-optimization properly propagates mibc with casts/isinst.

Also, I added "random class" stress mode support for it

[EgorBo]

@AndyAyersMS @jakobbotsch PTAL

[jakobbotsch]

Should this rather get its own config variable?

[EgorBo]

Just didn't want to produce even more variables 😄 and the default value for this one was OK to me.
in my other PR to add "multiple guesses" I slightly changed the whole logic so I'll leave it as is for now