[MemorySanitizer] Silence an unused-variable warning (NFC)

/llvm-project/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp:2622:22:
 error: unused variable 'ReturnType' [-Werror,-Wunused-variable]
    FixedVectorType *ReturnType = cast<FixedVectorType>(I.getType());
                     ^
1 error generated.

Author: DamonFool

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -2619,7 +2619,8 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
         cast<FixedVectorType>(I.getArgOperand(0)->getType());
     if (I.arg_size() == 2)
       assert(ParamType == cast<FixedVectorType>(I.getArgOperand(1)->getType()));
-    FixedVectorType *ReturnType = cast<FixedVectorType>(I.getType());
+    [[maybe_unused]] FixedVectorType *ReturnType =
+        cast<FixedVectorType>(I.getType());
     assert(ParamType->getNumElements() * I.arg_size() ==
            2 * ReturnType->getNumElements());