[dom] Add a basic dominator tree test.

[dom] Add a basic dominator tree test.

Correct, we have *zero* basic testing of the dominator tree in the
regression test suite. There is a single test that even prints it out,
and that test only checks a single line of the output. There are
a handful of tests that check post dominators, but all of those are
looking for bugs rather than just exercising the basic machinery.

This test is super boring and unexciting. But hey, it's something.
I needed there to be something so I could switch the basic test to run
with both the old and new pass manager.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225936 91177308-0d34-0410-b5e6-96231b3b80d8