[llvm] [IR][JumpThreading] Fix infinite recursion on compare self-reference [updated] (PR #129501)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

But `evaluateOnPredecessorEdge` recurses only into `CmpInst`s if I'm not mistaken?

When I run your example:
```
define void @foo() {
Preheader:
        br label %LoopHeader

LoopHeader:
        %common_phi = phi i32 [0, %Preheader], [%phi1, %Latch]
        br label %Latch

Latch:
        %phi1 = phi i32 [%common_phi, %LoopHeader]
        %phi2 = phi i32 [%common_phi, %LoopHeader]
        %cmp = icmp eq i32 %phi1, %phi2
        br i1 %cmp, label %LoopHeader, label %Exit

Exit:
        ret void
}
```
I get:
```
define void @foo() {
Preheader:
  br label %Latch

Latch:                                            ; preds = %Preheader, %Latch
  %common_phi = phi i32 [ 0, %Preheader ], [ %common_phi, %Latch ]
  %cmp = icmp eq i32 %common_phi, %common_phi
  br i1 %cmp, label %Latch, label %Exit

Exit:                                             ; preds = %Latch
  ret void
}
```
which should be what you intended?

https://github.com/llvm/llvm-project/pull/129501