[PATCH] SimplifyCFG: don't remove unreachable defaults from switch instructions; exploit them instead!

[Hans Wennborg]

>>! In D6471#14, @resistor wrote:
> Can you update LowerSwitch.cpp to incorporate this optimization?  Otherwise targets that rely on it are losing this functionality.

It seems LowerSwitch.cpp is already set up to exploit unreachable defaults, see the test in test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll

I guess it's a choice of whether that's important, or if moving popular cases into the default is better. If it's the latter I'm happy to implement it.

http://reviews.llvm.org/D6471