Store the shared ArrayMemoryPool in a field of its derived sealed type so the Jit can "see" the exact type when the MemoryPool<T>.Shared property is inlined which will allow it to devirtualize calls made on it.

Is a Roslyn proposal dotnet/roslyn#30797 by @stephentoub where field initalizer, backing field and property could be combined via { get } = to support devirtualization by having the auto-backing field be created as the derived type rather than the exposed type.

Similar scenario to ArrayPool<T>.Shared dotnet/coreclr#20637

@AndyAyersMS

var mem = MemoryPool<byte>.Shared.Rent();

might be a double devirt inline "opportunity" as MemoryPool<T>.Shared.Rent is

MemoryPool<T>.Shared.Rent
=> ArrayMemoryPool<T>.Rent
=> new ArrayMemoryPoolBuffer
=> ArrayPool<T>.Shared.Rent
=> TlsOverPerCoreLockedStacksArrayPool<T>.Rent

/cc @ahsonkhan @jaredpar