Use local array in ConcurrentQueue<T> for small perf improvement (#21192)

Use local array in ConcurrentQueue<T> for small perf improvement (#21192)

Enqueue/TryDequeue/TryPeek are all repeatedly accessing the same array from a readonly field.  Changing them to instead access that same array but from a cached local results in an ~10% throughput boost on a microbenchmark that does uncontended reads/writes of objects from/to the queue. (I also tried using a ref local to point directly to the target slot in the array, but that actually resulted in a measurable regression.)