Fix Debug.Assert in GenericDelegateCache

[safern]

The Debug.Assert is not matching what we're casting the value to, nor the generic parameters. Causing the JIT Perf tests to fail in the PR to merge to master:

Assertion Failed

   at System.Threading.Tasks.GenericDelegateCache`2.<>c.<.cctor>b__4_2(Task`1 wrappedAntecedents, Object state)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.<>c.<.cctor>b__274_0(Object obj)
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
   at System.Threading.Tasks.Task.ExecuteEntryUnsafe(Thread threadPoolThread)
   at System.Threading.Tasks.Task.ExecuteFromThreadPool(Thread threadPoolThread)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

cc: @stephentoub @danmosemsft