Fixed build warning. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76761 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Writer/ValueEnumerator.cpp

@@ -265,7 +265,7 @@ void ValueEnumerator::EnumerateOperandType(const Value *V) {
           EnumerateOperandType(Elem);
       }
     }
-  } else if (const MDString *MDS = dyn_cast<MDString>(V))
+  } else if (dyn_cast<MDString>(V))
     EnumerateValue(V);
 }