[SimplifyCFG] Prune code from a provably unreachable switch default

As Sanjoy pointed out over in http://reviews.llvm.org/D11819, a switch on an icmp should always be able to become a branch instruction. This patch generalizes that notion slightly to prove that the default case of a switch is unreachable if the cases completely cover all possible bit patterns in the condition. Once that's done, the switch to branch conversion kicks in just fine.

Note: Duplicate case values are disallowed by the LangRef and verifier.

Differential Revision: http://reviews.llvm.org/D11995

llvm-svn: 246125