script improvements...

- support a user-defined tag for cache entries so that cache-infer
  can remember what it has accomplished if it gets killed or dies

- don't parse all LHSs by default, this was slow and wasn't turning
  up problems

- be verbose, if asked
Merge pull request #132 from regehr/cache-dump-improvements

script improvements
Merge remote-tracking branch 'origin/master' into simple-synthesis
teach the pass to use the PostDominatorTree analysis

ExtractorTests needed a bit of help dealing with the harvesting
of non-i1s
Merge branch 'simple-synthesis' of github.com:regehr/souper into simple-synthesis