[llvm] [NewGVN] Prevent cyclic reference when building phiofops (PR #69418)

[via llvm-commits]

XChy wrote:

I tried performing SCC finding starting from the leader of `OrigInst`.
```C++
  TarjanSCC CycleFinder;
  DenseMap<const Value *, SmallVector<Value *, 4>> Edges;
  Value *OrigInstLeader =
      lookupOperandLeader(const_cast<Value *>((const Value *)OrigInst));
  auto *LeaderInst = dyn_cast<Instruction>(OrigInstLeader);

  SmallVector<Value *, 4> Members(CC->begin(), CC->end());
  Edges[OrigInst] = Members;

  CycleFinder.Start(OrigInst, Edges);
  if (LeaderInst) {
    Edges[LeaderInst] = Members;
    CycleFinder.Start(LeaderInst, Edges);
  }
```
But it would lead to the fact that we could possibly optimize the IR after eliminate all the redundancies. Example:
```llvm
define i64 @test5() {
bb:
  %tmp10 = load i64, ptr null, align 16
  %tmp12 = mul i64 %tmp10, %tmp10
  %tmp13 = icmp eq i64 %tmp12, 0
  br label %bb15

bb15:                                             ; preds = %bb15, %bb
  %tmp16 = phi i64 [ 0, %bb15 ], [ %tmp10, %bb ]
  %tmp19 = mul i64 %tmp16, %tmp10
  store i64 %tmp19, ptr null, align 8
  br label %bb15
}
```

https://github.com/llvm/llvm-project/pull/69418