[mlir][NFC] Use .empty() instead of .size()

mlir/lib/Dialect/Affine/Utils/Utils.cpp

@@ -591,7 +591,7 @@ static bool hasNoInterveningEffect(Operation *start, T memOp) {
 
         SmallPtrSet<Block *, 4> done;
         // Traverse the CFG until hitting `to`.
-        while (todoBlocks.size()) {
+        while (!todoBlocks.empty()) {
           Block *blk = todoBlocks.pop_back_val();
           if (done.count(blk))
             continue;