Added CFG support for: for loops

In CFG dumper, refactored the code to print block terminators into a
StmtVisitor.

Added the method "FinishBlock" to CFGBuilder to do the reversal of statements
in a block instead of calling "reverseStmts" for a block directly.  This
was necessary to fix a bug in how blocks with labels were being processed
(some cases would cause the statements to be reversed twice).  FinishBlock
detects blocks that start with labels and doesn't do a second reversal.

llvm-svn: 41281