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/MachineInstr.cpp

@@ -221,6 +221,7 @@ bool MachineOperand::isIdenticalTo(const MachineOperand &Other) const {
   case MachineOperand::MO_Metadata:
     return getMetadata() == Other.getMetadata();
   }
+  llvm_unreachable("Invalid machine operand type");
 }
 
 /// print - Print the specified machine operand.