change the serialized souper format a bit as suggested by Peter in email:
annotate each block with its number of predecessors; this simplifies parsing a
bit
oops this slipped through my earlier patch to remove isTriviallyInvalid
Merge pull request #109 from regehr/blockpred

change the serialized souper format a bit as suggested by Peter in email
Merge pull request #113 from regehr/cleanup-triviallyinvalid

oops this slipped through my earlier patch to remove isTriviallyInvalid