Investigate benefits of separating out parts of Count into a non-generic method.

[jamesqo]

Separated out from dotnet/corefx#12703. Relevant discussion: dotnet/corefx#12703 (comment)

tl;dr We may want to put a part of Enumerable.Count<T> in a non-generic method so that the JIT has less code to generate during the following 2 scenarios:

• For a particular value type T, the source enumerable always implements ICollection, IIListProvider, etc. and we can determine the count without actually enumerating

• We fall back to iterating manually over the enumerable for at least 2 different value types

More details can be found in the comment linked to. Opening this as a tracking issue.

[stephentoub]

This isn't actionable.