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.

llvm-svn: 103184