Blockpc

This patch addresses issue #81 by implementing blockpc based on Peter's
suggestion. blockpc has the same semantics as pc, except that pc only applies
when the given predecessor of the given block is chosen.

blockpc(s) are collected in a way similar to what we currently do for pc(s),
and are encoded with a similar technique for encoding UB constraints.

The parser is also updated to parse blockpc(s).

I am sorry the path is a bit big...

This is not the representation I would choose for a blockpc. I would just store a tuple of (Block *B, unsigned Index, InstMapping Mapping). This would allow you to have any number of predicates for each predecessor.