Changes:
- introduce the notion of unreachable abstract states.

- reconnect unreachables states to runtime abort in effect phis (so that
  the merged states are not polluted by unreachable branches while
  preserving SSA).

- mark states with failed map checks, unreachable map guars as unreachable.

- add instance type to AbstractMaps, only invalidate instance type on
  mismatched effect merges.


This results in 2-3% improvement on ARES/ML steady state.

Bug: v8:6396
Change-Id: I35b0d4482fa400ba7ee9a754f8ef1b2663ebc7dc
Reviewed-on: https://chromium-review.googlesource.com/727761Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48742}