Suppress warning for unused variable if assertions are disabled.

mlir/lib/Rewrite/ByteCode.cpp

void PDLByteCode::match(Operation *op, PatternRewriter &rewriter,
   // The matcher function always starts at code address 0.
   ByteCodeExecutor executor(
       matcherByteCode.data(), state.memory, state.opRangeMemory,
       state.typeRangeMemory, state.allocatedTypeRangeMemory,
       state.valueRangeMemory, state.allocatedValueRangeMemory, state.loopIndex,
       uniquedData, matcherByteCode, state.currentPatternBenefits, patterns,
       constraintFunctions, rewriteFunctions);
   LogicalResult executeResult = executor.execute(rewriter, &matches);
+  (void)executeResult;
   assert(succeeded(executeResult) && "unexpected matcher execution failure");
 
   // Order the found matches by benefit.
   std::stable_sort(matches.begin(), matches.end(),
                    [](const MatchResult &lhs, const MatchResult &rhs) {
                      return lhs.benefit > rhs.benefit;
                    });
 }