SimplifyCFG currently does this transformation, but I'm planning to remove that
to allow other passes, such as this one, to exploit the unreachable default.
This patch takes care to keep track of what case values are unreachable even
after the transformation, allowing for more efficient lowering.
Differential Revision: http://reviews.llvm.org/D6697
llvm-svn: 226934
Hans Wennborg