[mlir] Fixed warning related to unused variable in Liveness.

[dfki-mako]

Summary

The latest changes of the Liveness analysis caused a warning related to an unused variable. This commit solves this warning.

[herhut]

Thanks!

mlir/lib/Analysis/Liveness.cpp

for (Operation &operation : block) {
       operationIds.insert({&operation, operationIds.size()});
       for (Value result : operation.getResults())
         valueIds.insert({result, valueIds.size()});
     }
   });
 
   // Local printing helpers
   auto printValueRef = [&](Value value) {
-    if (Operation *defOp = value.getDefiningOp())
+    if (value.getDefiningOp())
       os << "val_" << valueIds[value];
     else {
       auto blockArg = value.cast<BlockArgument>();
       os << "arg" << blockArg.getArgNumber() << "@"
          << blockIds[blockArg.getOwner()];
     }
     os << " ";
   };