Added extensions (subclasses) to the StmtVisitor class that

are useful for dataflow analysis: CFGStmtVisitor and DataflowStmtVisitor.

CFGStmtVisitor is the same as StmtVisitor is that it has separate visitors
for "root" statements in a CFGBlock (statements that have a designated
slot int the list of statements in a CFGBlock).  It also recognizes statements
that have implicit control-flow, and calls special visitor methods for those.

DataflowStmtVisitor extends CFGStmtVisitor to serve as a template for
implementing transfer functions.  It does a pre-/post-order traversal of
substatements depending on whether we are doing a forward/backward analysis.
It also has special handling for implicit-control-flow statements so that
they are visited only once.

llvm-svn: 41884