Rewrite BitVector index iterator.

The BitVector::Iterator was not iterating over the bits but
rather over indexes of the set bits. Therefore, we rename it
to IndexIterator and provide a BitVector::Indexes() to get
a container-style interface with begin() and end() for range
based for loops.

Also, simplify InsertPhiNodes where the tmp_blocks isn't
needed since the phi_nodes and input_blocks cannot lose any
blocks in subsequent iterations, so we can do the Union()
directly in those bit vectors and we need to repeat the loop
only if we have new input_blocks, rather than on phi_nodes
change. And move the temporary bit vectors to scoped arena.

Change-Id: I6cb87a2f60724eeef67c6aaa34b36ed5acde6d43