[Triton-MLIR][Backend] Remove unnecessary barriers (#901)

Cross operation barriers are taken care of by the Membar pass. 

Explicit barriers are only required if there's any synchronization
necessary within each operation.

lib/Analysis/Membar.cpp

@@ -86,9 +86,10 @@ void MembarAnalysis::transfer(Operation *op, RegionInfo *regionInfo,
       curRegionInfo.syncWriteBuffers.insert(bufferId);
     }
   }
-  // Scratch buffer is considered as a shared memory read
+  // Scratch buffer is considered as both shared memory write & read
   auto bufferId = allocation->getBufferId(op);
   if (bufferId != Allocation::InvalidBufferId) {
+    curRegionInfo.syncWriteBuffers.insert(bufferId);
     curRegionInfo.syncReadBuffers.insert(bufferId);
   }