var castParam1 = Expression.Variable(...);
...
var blockMembers = new Expression[]
{
    Expression.Assign(castParam1, unsafeAs),
    Expression.Assign(castParam2, unsafeAs),
    comparisonsConjuncted,
};
var block = Expression.Block(new[] { castParam1, castParam2 }, blockMembers);
var lambda = Expression.Lambda<...>(block, new[] { param1, param2 });

    private static AsmGetHashCodeDelegate CreateHashCodeFunc(Type asmType)
    {
        var getTypeMethod = asmType.GetMethod(nameof(GetType))!;
        var toHashCodeMethod = typeof(HashCode).GetMethod(nameof(HashCode.ToHashCode))!;

        var param = Expression.Parameter(typeof(IAsyncStateMachine));

        var hashCode = Expression.Variable(typeof(HashCode));
        var asmAsConcreteType = Expression.Variable(asmType);

        var blockExprs = new List<Expression>();
        blockExprs.Add(Expression.Assign(hashCode, Expression.Default(typeof(HashCode))));
        blockExprs.Add(Expression.Assign(asmAsConcreteType, CastToConcreteType(param, asmType)));

        foreach (var field in GetRelevantFields(asmType))
        {
            blockExprs.Add(Expression.Call(
                instance: hashCode,
                methodName: nameof(HashCode.Add),
                typeArguments: new[] { field.FieldType },
                arguments: Expression.MakeMemberAccess(asmAsConcreteType, field)));
        }

        blockExprs.Add(Expression.Call(
            instance: hashCode,
            method: toHashCodeMethod));

        var block = Expression.Block(blockExprs);
        var lambda = Expression.Lambda<Func<IAsyncStateMachine, int>>(block, param);

        return new(lambda.Compile());
    }

System.InvalidOperationException: 'variable '' of type 'System.HashCode' referenced from scope '', but it is not defined'

You need to give it the list of variables used in the block as well