[mlir] Recover the behavior of SliceAnaylsis for 6a8dde0

In 6a8dde0,
it changes the method to return LogicalFailure, so callers can handle
the failure instead of crashing, if I read the intention correctly.
However, it changes the behavior of the implementation.

Before the change, processValue does not treat it as a failure if the
check below TODO has a false condition. However, with the new change, it
starts treating it as a failure.

The revision updates the final `else` branch to return a failure, and
return success at the end; the behavior is recovered.

```cpp
auto processValue = [&](Value value) {
  if (auto *definingOp = value.getDefiningOp()) {
    if (backwardSlice->count(definingOp) == 0)
      getBackwardSliceImpl(definingOp, backwardSlice, options);
  } else if (auto blockArg = dyn_cast<BlockArgument>(value)) {
    if (options.omitBlockArguments)
      return;

    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 &&
             llvm::hasSingleElement(parentOp->getRegion(0).getBlocks()));
      getBackwardSliceImpl(parentOp, backwardSlice, options);

    }
  } else {
    llvm_unreachable("No definingOp and not a block argument.");
  }
```

No additional tests are added like the previous commit. This revision is
mostly a post-fix for 6a8dde0

Signed-off-by: hanhanW <[email protected]>

Author: hanhanW

mlir/lib/Analysis/SliceAnalysis.cpp

@@ -111,8 +111,10 @@ static LogicalResult getBackwardSliceImpl(Operation *op,
           return getBackwardSliceImpl(parentOp, backwardSlice, options);
         }
       }
+    } else {
+      return failure();
     }
-    return failure();
+    return success();
   };
 
   bool succeeded = true;