[Vectorize] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:855:52: error:
  unused variable 'SupportedOp' [-Werror,-Wunused-const-variable]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -876,6 +876,7 @@ class BinOpSameOpcodeHelper {
   isBinOpWithConstantInt(Instruction *I) {
     unsigned Opcode = I->getOpcode();
     assert(binary_search(SupportedOp, Opcode) && "Unsupported opcode.");
+    (void)SupportedOp;
     auto *BinOp = cast<BinaryOperator>(I);
     if (auto *CI = dyn_cast<ConstantInt>(BinOp->getOperand(1)))
       return {CI, 1};