[CodeGen] Omit range checks from jump tables when lowering switches with unreachable default

During the lowering of a switch that would result in the generation of a 
jump table, a range check is performed before indexing into the jump 
table, for the switch value being outside the jump table range and a 
conditional branch is inserted to jump to the default block. In case the 
default block is unreachable, this conditional jump can be omitted. This 
patch implements omitting this conditional branch for unreachable 
defaults.

Review ID: D52002
Reviewers: Hans Wennborg, Eli Freidman, Roman Lebedev

llvm-svn: 352484