Fix a nasty problem with dominance calculation for unreachable blocks.

Fix a nasty problem with dominance calculation for unreachable blocks.
If we had a CFG that look like Entry -> B, Unreachable -> B, then we would
not correctly determine that Entry dominated B, because Entry did not
apparently dominate "unreachable".  This patch fixes this by making the entry
node dominate all blocks, including unreachable ones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4037 91177308-0d34-0410-b5e6-96231b3b80d8