Fix bad CFG construction bug when handling C++ 'try' statements.

This code assigned the last created CFGBlock* to the variable 'Block',
which is a scratch variable which is null'ed out after a block is
completed.  By assigning the last created block to 'Block', we start
editing a completed block, inserting CFGStmts that should be in
another block.  This was the case with 'try'.  The test case that
showed this had a while loop inside a 'try', and the logic before
the while loop was being included as part of the "condition block"
for the loop.  This showed up as a bogus dead store, but could
have lots of implications.

Turns out this bug was replicated a few times within CFG.cpp, so
I went and fixed up those as well.

llvm-svn: 167788