class SyntaxTree::YARV::ControlFlowGraph::Compiler

def prune_basic_blocks(blocks)

graph entirely at this point.
If there are blocks that are unreachable, we can remove them from the 
def prune_basic_blocks(blocks)
  visited = Set.new
  queue = [blocks.fetch(0)]
  until queue.empty?
    current_block = queue.shift
    next if visited.include?(current_block)
    visited << current_block
    queue.concat(current_block.outgoing_blocks)
  end
  blocks.select! { |_, block| visited.include?(block) }
end