[JumpThreading] NFC: Simplify ComputeValueKnownInPredecessorsImpl

Summary:
ComputeValueKnownInPredecessorsImpl is the main folding mechanism in
JumpThreading.cpp.  To avoid potential infinite recursion while
chasing use-def chains, it uses:

  DenseSet<std::pair<Value *, BasicBlock *>> &RecursionSet

to keep track of Value-BB pairs that we've processed.

Now, when ComputeValueKnownInPredecessorsImpl recursively calls
itself, it always passes BB as is, so the second element is always BB.

This patch simplifes the function by dropping "BasicBlock *" from
RecursionSet.

Reviewers: wmi, efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77699