[CodeGen] Add an -O0 backend pipeline test. NFC.

This should hopefully makes changes to the O0 pipeline obvious; it's
easy to require expensive passes, and this helps make informed
decisions.

Case in point: in the few weeks separating the time when I initially
wrote this patch to the time when I committed, the test regressed as
r302103 added another use of DT!

llvm-svn: 302608