The problem is that because of C++ integral promotion rules, many of the i8 and i16 arithmetic tests were actually being performed as i32 operations.  Thus we weren't actually testing everything we meant to test.

The fix is to have a python script auto-generate the relevant tests with proper typing.

BUG= none
R=eholk@chromium.org, jpp@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/2013863002 .