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

[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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302608 91177308-0d34-0410-b5e6-96231b3b80d8