The problem is that the memory usage that comes from the -track-memory option is not very well-defined (it's a hidden LLVM option after all).  It gives an OK sense of memory growth over time, but sometimes we really want to know how much CFG-local arena memory was allocated for a particular function.

To help with this, we add another row to the stats output, giving the MB size of the CFG arena at the end of translating the method.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1848733002 .