[mlir] AsyncRefCounting: check that LivenessBlockInfo is not nullptr

mlir/lib/Dialect/Async/Transforms/AsyncRuntimeRefCounting.cpp

AsyncRuntimeRefCountingPass::addDropRefInDivergentLivenessSuccessor(
   // `value` we can only process CFG for the same region.
   Region *definingRegion = value.getParentRegion();
 
   // Collect blocks with successors with mismatching `liveIn` sets.
   for (Block &block : definingRegion->getBlocks()) {
     const LivenessBlockInfo *blockLiveness = liveness.getLiveness(&block);
 
     // Skip the block if value is not in the `liveOut` set.
-    if (!blockLiveness->isLiveOut(value))
+    if (!blockLiveness || !blockLiveness->isLiveOut(value))
       continue;
 
     BlockSet liveInSuccessors;   // `value` is in `liveIn` set
     BlockSet noLiveInSuccessors; // `value` is not in the `liveIn` set
 
     // Collect successors that do not have `value` in the `liveIn` set.
     for (Block *successor : block.getSuccessors()) {
       const LivenessBlockInfo *succLiveness = liveness.getLiveness(successor);
-      if (succLiveness->isLiveIn(value))
+      if (succLiveness && succLiveness->isLiveIn(value))
         liveInSuccessors.insert(successor);
       else
         noLiveInSuccessors.insert(successor);
     }
 
     // Block has successors with different `liveIn` property of the `value`.
     if (!liveInSuccessors.empty() && !noLiveInSuccessors.empty())
       divergentLivenessBlocks.try_emplace(&block, noLiveInSuccessors);