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

[via llvm-commits]

XChy wrote:

> For example, be an operand of one of the operands of the original instruction. Or an even more contrived case would be if one of the operands of the leader was the leader of one of the operands of the original instruction. I don't have any concrete examples of this happening and causing NewGVN to not terminate

For the first case, I think that's correct. I missed here. There is a case:
```llvm
define i64 @foo(i64 %e) {
entry:
  br label %for.body

for.body:                                        ; preds = %for.body, %entry
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
  %original = add i64 %indvars.iv, 1

  %b = add i64 %original, 1
  %c = sub i64 %b, 1

  %phileader = add i64 %c, 1

  %indvars.iv.next = add i64 %indvars.iv, 1
  %tobool = icmp slt i64 %indvars.iv.next, 100
  br i1 %tobool, label %for.body, label %end

end:
  ret i64 %phileader
}```
But for the latter, I think that's OK. Based on my knowledge about this algorithm, if operand of `phiopsleader` is equivalent/congruent to the operand of `orig`. For example(May not so precise):
```llvm
declare void @use(i64)
define i64 @foo(i64 %operand_lead) {
entry:
  br label %for.body

for.body:                                        ; preds = %for.body, %entry
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]

  %operand_mid = add i64 %operand_lead, 1
  %operand = sub i64 %operand_lead, 1
  %original = add i64 %indvars.iv, %operand
  %phi_leader = add i64 %indvars.iv, %operand_lead

  call void @use(i64 %original)
  call void @use(i64 %phi_leader)

  %indvars.iv.next = add i64 %indvars.iv, %operand

  %tobool = icmp slt i64 %indvars.iv.next, 100
  br i1 %tobool, label %for.body, label %end

end:
  ret i64 0
}
```
It would be fine for me.

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