[JumpThreading] Change a return of ComputeValueKnownInPredecessors()

Change a return statement of ComputeValueKnownInPredecessors() to be the same as
the rest return statements of the function. Otherwise, it might return true with
an empty Result when the current basic block has no predecessors and trigger the
first assert of JumpThreading::ProcessThreadableEdges().

llvm-svn: 260110

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -415,7 +415,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
     for (BasicBlock *Pred : predecessors(BB))
       Result.push_back(std::make_pair(KC, Pred));
 
-    return true;
+    return !Result.empty();
   }
 
   // If V is a non-instruction value, or an instruction in a different block,