[Vectorize] Fix -Wunused-variable in SLPVectorizer.cpp (NFC)

/llvm-project/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:10310:26:
error: unused variable 'isExtractSubvectorMask' [-Werror,-Wunused-variable]
                    bool isExtractSubvectorMask =
                         ^
1 error generated.

Author: DamonFool

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -10307,7 +10307,7 @@ BoUpSLP::getEntryCost(const TreeEntry *E, ArrayRef<Value *> VectorizedVals,
                            "Not supported shufflevector usage.");
                     auto *SV = cast<ShuffleVectorInst>(V);
                     int Index;
-                    bool isExtractSubvectorMask =
+                    [[maybe_unused]] bool isExtractSubvectorMask =
                         SV->isExtractSubvectorMask(Index);
                     assert(isExtractSubvectorMask &&
                            "Not supported shufflevector usage.");