[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