[PATCH] [analyzer] Fix inefficiency in dead symbol removal

[Jordan Rose]

  This optimization is not correct -- the region has been visited for the //present// store, but possibly not in the //past// store (the snapshot store in the LazyCompoundVal), and it may have different values at each point in "time". We could make this work by making the visited set use a (region, store) pair, though.

http://llvm-reviews.chandlerc.com/D1398