[mlir] Move the assertion to a valid place.

[hanchung]

Summary

The defining Op may live in an unlinked block so its parent Op may be
null. Only assert it when the parent Op is not null.

mlir/lib/Analysis/SliceAnalysis.cpp

if (auto *definingOp = operand.getDefiningOp()) {
       if (backwardSlice->count(definingOp) == 0)
         getBackwardSliceImpl(definingOp, backwardSlice, filter);
     } else if (auto blockArg = operand.dyn_cast<BlockArgument>()) {
       Block *block = blockArg.getOwner();
       Operation *parentOp = block->getParentOp();
       // TODO: determine whether we want to recurse backward into the other
       // blocks of parentOp, which are not technically backward unless they flow
       // into us. For now, just bail.
+      if (parentOp && backwardSlice->count(parentOp) == 0) {
         assert(parentOp->getNumRegions() == 1 &&
                parentOp->getRegion(0).getBlocks().size() == 1);
-      if (backwardSlice->count(parentOp) == 0)
         getBackwardSliceImpl(parentOp, backwardSlice, filter);
+      }
     } else {
       llvm_unreachable("No definingOp and not a block argument.");
     }
   }
 
   backwardSlice->insert(op);
 }