[mlir][sparse] fix build issue with unused local under opt builds

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

if (codegen.redVal) {
     return;
   }
   // Store during insertion.
   OpOperand *t = op.getOutputOperand(0);
   if (t == codegen.sparseOut) {
     if (!rhs) {
       // Only unary and binary are allowed to return uninitialized rhs
       // to indicate missing output.
-      Kind kind = merger.exp(exp).kind;
-      assert(kind == kUnary || kind == kBinary);
+      assert(merger.exp(exp).kind == kUnary || merger.exp(exp).kind == kBinary);
     } else {
       genInsertionStore(codegen, rewriter, op, t, rhs);
     }
     return;
   }
   // Actual store.
   SmallVector<Value, 4> args;
   Value ptr = genSubscript(codegen, rewriter, op, t, args);