Silence variable set but not used warning in CodeGenRegisters.cpp, NFC.

llvm-svn: 230854

llvm/utils/TableGen/CodeGenRegisters.cpp

@@ -1783,6 +1783,7 @@ void CodeGenRegBank::computeRegUnitLaneMasks() {
           }
           ++u;
         }
+        (void)Found;
         assert(Found);
       }
     }