Fix handling of unreachable blocks in the SSAUpdater. The previous code only

Fix handling of unreachable blocks in the SSAUpdater.  The previous code only
handled cases where a block had zero predecessors, but failed to detect other
cases like loops with no entries.  The SSAUpdater is already doing a forward
traversal through the blocks, so it is not hard to identify the blocks that
were never reached on that traversal.  This fixes the crash for ppc on the
stepanov_vector test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103184 91177308-0d34-0410-b5e6-96231b3b80d8