code: private BitSet findPreviouslyDeadBlocks() throws DataflowAnalysisException, CFGBuilderException {
          BitSet deadBlocks = new BitSet();
          ValueNumberDataflow vnaDataflow = classContext.getValueNumberDataflow(method);
          for (Iterator<BasicBlock> i = vnaDataflow.getCFG().blockIterator(); i.hasNext();) {
              BasicBlock block = i.next();
              ValueNumberFrame vnaFrame = vnaDataflow.getStartFact(block);
              if (vnaFrame.isTop()) {
                  deadBlocks.set(block.getLabel());
              }
          }
  
          return deadBlocks;
      }
docs: Find set of blocks which were known to be dead before doing the null
  pointer analysis.
  
  @return set of previously dead blocks, indexed by block id
  @throws CFGBuilderException
  @throws DataflowAnalysisException