[llvm] [ValueTracking] Skip incoming values that are the same as the phi in `isGuaranteedNotToBeUndefOrPoison` (PR #130111)

[Nikita Popov via llvm-commits]

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nikic wrote:

I had something like this in mind:
```
declare i32 @opaque()

define i32 @other_noundef() {
start:
  br label %loop

loop:
  %nonpoison = phi i32 [ 0, %start ], [ %nonpoison, %bb0 ], [ 1, %bb1 ]
  %i = call i32 @opaque()
  switch i32 %i, label %exit [
  i32 0, label %bb0
  i32 1, label %bb1
  ]

exit:
  %r = freeze i32 %nonpoison
  ret i32 %r

bb0:
  br label %loop

bb1:
  br label %loop
}
```

Avoids the need for an extra block for the phi.

https://github.com/llvm/llvm-project/pull/130111