Update the documentation on how the packing transformation is implemented

Add a simple example to update the documentation on how the packing
transformation is implemented.

Reviewed-by: Tobias Grosser <tobias@grosser.es&gt;,>
             Michael Kruse <llvm@meinersbur.de>

Differential Revision: https://reviews.llvm.org/D28021

llvm-svn: 293429