[dataflow] HTMLLogger: fix off-by-one in the BB listing

The indexes of analysis state within a BB element is a bit odd:
  BB.0 is the initial state
  BB.1 is the state after the first element
etc

This means we have N+1 states and we need N+1 elements in the BB list.
We add a dummy element at the beginning rather than the end, because we want
selecting a CFG element to show the state *afterwards*.
For example, if we click on an expr, we want to be able to see its value model!

Differential Revision: https://reviews.llvm.org/D148951