Fix dominator descendants for unreachable blocks.

When a block is unreachable, asking its dom tree descendants should
return the empty set. However, the computation of the descendants
was causing a segmentation fault because the dom tree node we get
from the basic block is initially NULL.

Fixed by adding a test for a valid dom tree node before we iterate.

The patch also adds some unit tests to the existing dom tree tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196099 91177308-0d34-0410-b5e6-96231b3b80d8