LowerSwitch: replace unreachable default with popular case destination

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