[ARM] Add a first test for the Thumb-2 code size optimization pass.

While working on a Thumb-2 code size optimization I just realized that we don't have any regression tests for it.

So here's a first test case, I plan to increase the coverage over time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216728 91177308-0d34-0410-b5e6-96231b3b80d8