Remove unreachable code. (replace with llvm_unreachable to help GCC where necessary)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148284 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-mc/llvm-mc.cpp

@@ -526,6 +526,4 @@ int main(int argc, char **argv) {
   case AC_EDisassemble:
     return DisassembleInput(argv[0], true);
   }
-
-  return 0;
 }