backport causal fix (#698)

Author: njriasan

lib/Dialect/TritonGPU/Transforms/Utility.cpp

@@ -1212,6 +1212,10 @@ struct ForOpDeadArgElimination : public OpRewritePattern<scf::ForOp> {
         auto result = mlir::cast<OpResult>(value);
         OpOperand &forOperand = *nestedFor.getTiedLoopInit(result);
         markLive(forOperand.get());
+        // Mark the lowerBound, upperBound, and step as live.
+        markLive(nestedFor.getLowerBound());
+        markLive(nestedFor.getUpperBound());
+        markLive(nestedFor.getStep());
         auto nestedYieldOp =
             cast<scf::YieldOp>(nestedFor.getBody()->getTerminator());
         Value nestedYieldOperand =