Avoid unnecessary work for identical locations in Buffer.BlockCopy

Avoid unnecessary work for identical locations in Buffer.BlockCopy

Perform validity checks to ensure parameters are correct but short-circuit
out memmove when exactly the same data would be copied to the same location.

There are a number of occasions; which can be intentional or unintentional,
where the buffer being copied is the same place - e.g an internal buffer is
the same as the return buffer, and there is no need to call memmove's
overwrite safe copy.

Generally the call to BlockCopy will be in a library so it is more practical
to enable the check here rather than alter all the calling functions,
including 3rd party libraries to preform additional checks.