[cmake] Enable reverse iteration by default through build macro

Summary:
Reverse iteration can be turned on, by default, by setting -DLLVM_REVERSE_ITERATION:BOOL=ON during cmake.
With this enabled, we can uncover lots of cases of non-determinism in codegen by simply running our tests (without any other change).
We can then setup a buildbot which will have this turned on by default. Initially, a lot of unit tests will fail in this configuration.
Once we start fixing non-determinism issues, we can gradually make this a blocker for patches.

Reviewers: davide, dblaikie, mehdi_amini, dberlin

Reviewed By: dblaikie

Subscribers: probinson, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D33908

llvm-svn: 304757