Further refactored DataflowSolver. Now most code for the solver is shared

between forward and backward analyses, with trait classes being used
to implement the key differences in operations/functionality.

Converted the LiveVariables analysis to use the generic DataflowSolver.  This,
along with removing some extra functionality that was not needed, reduced
the code for LiveVariables by over half.

Modified Driver code to handle the updated interface to LiveVariables.

Modified the DeadStores checker to handle the update interface to
LiveVariables.

Updated DataflowValues (generic ADT to store dataflow values) to also
store values for blocks.  This is used by DeadStores.  Updated some comments.

llvm-svn: 42293