Add 'llvm_unreachable' to passify GCC's understanding of the constraints

of several newly un-defaulted switches. This also helps optimizers
(including LLVM's) recognize that every case is covered, and we should
assume as much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147861 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LLVMTargetMachine.cpp

@@ -96,6 +96,7 @@ static bool getVerboseAsm() {
   case cl::BOU_TRUE:  return true;
   case cl::BOU_FALSE: return false;
   }
+  llvm_unreachable("Invalid verbose asm state");
 }
 
 // Enable or disable FastISel. Both options are needed, because