[mlir][sparse] Fixing -Wunused-variable in Sparsification.cpp

[wrengr]

Note for posterity: this warning seems to have been introduced by the rebase when landing D146086

mlir/lib/Dialect/SparseTensor/Transforms/Sparsification.cpp

static bool computeIterationGraph(CodegenEnv &env, SortMask mask,
   // for the implicit loop indices i_0 .. i_n-1.
   const LoopId n = env.merger().getNumLoops();
   std::vector<std::vector<bool>> adjM(n, std::vector<bool>(n, false));
   std::vector<unsigned> inDegree(n, 0); // in-degree of each node.
   const auto iteratorTypes = env.op().getIteratorTypesArray();
   // Iterate over the indexing maps of every tensor in the tensor expression.
   for (OpOperand &t : env.op()->getOpOperands()) {
     // Get map and encoding.
-    const auto map = env.op().getMatchingIndexingMap(&t);
     const auto enc = getSparseTensorEncoding(t.get().getType());
-    assert(map.getNumDims() + getNumNonTrivialIdxExpOnSparseLvls(env.op()) ==
+    assert(env.op().getMatchingIndexingMap(&t).getNumDims() +
+               getNumNonTrivialIdxExpOnSparseLvls(env.op()) ==
            n);
 
     // Skips dense inputs/outputs when not requested.
     const bool isDenseInput = !enc && env.op().isDpsInput(&t);
     const bool isDenseOutput = !enc && !isDenseInput;
     if ((isDenseInput && !includesDenseInput(mask)) ||
         (isDenseOutput && !includesDenseOutput(mask)))
       continue;