[IR] Bug fix in CFG - reset cached state when a new BB is added

* We need a better solution than caching. Added a FIXME to revisit
  this in the near future as we do another round of cleanup in
  how we build IR and manage state.

* This fixes the bug where all instrs of the global ensure block
  were being (incorrectly) marked dead.

  jruby -X_CIR -Xir.passes='AddLocalVarLoadStoreInstructions,AddCallProtocolInstructions,DeadCodeElimination,LinearizeCFG' -e 'p 1'