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...

Seems that harvesting pcs and blockpcs separately makes the code logic clean and easy to follow. Otherwise, we would add check in addPathConditions to examine when to add conditions to pcs and when to add conditions to blockpcs.  It adds complexity to the function.