[llvm] [Instcombine]: Folds`llvm.ucmp` and `llvm.scmp` (PR #168505)

[Kevin Per via llvm-commits]

kper wrote:

> The remaining transform here looks like something that the select equivalence class of transforms should handle. I think the reason it doesn't is potential derefinement, which requires adding these intrinsics as a special case in

hmmm
Putting it into `simplifyWithOpsReplaced` requires a bit of refactoring and double checking. 

At the moment, `simplifyWithOpsReplaced` is only called in `simplifySelectWithEquivalence`.
`simplifySelectWithEquivalence` only replaces an instruction when both simplified true and simplified false return the same value. However, that's not true for that transformation. The simplified true val must be a constant and I would need to double check whether the simplified false val is an intrinsic. Something that I would have already done in `simplifyWithOpsReplaced` (hence double checking).

https://github.com/llvm/llvm-project/blob/8290134a7421c5a5ce3348b29f908887158b4b8a/llvm/lib/Analysis/InstructionSimplify.cpp#L4690

In the latest commit, I added a method `simplifySelectWhenValInstrinsic` that tries to fold the computation after the existing implementation is done.

---

> Something to watch out for are poison-generating attributes on the call. I believe your current implementation is also not strictly correct in that regard (e.g. imagine that there's a range(1, 2) attribute on the return value, but after the transform a return of 0 is also valid.)

Could you give me an example where this range attribute could be put? I tried the snippet below but `opt` is correcting the range for me automatically.

Input:
```
; HERE is the range updated
declare range(i32 1, 2) i32 @llvm.ucmp.i32.i32(i32, i32) #0

define i32 @fold_ucmp(i32 %0, i32 %1) {
  %3 = icmp eq i32 %0, %1
  %4 = tail call i32 @llvm.ucmp.i32.i32(i32 %0, i32 %1)
  %5 = select i1 %3, i32 0, i32 %4
  ret i32 %5
}
```

https://github.com/llvm/llvm-project/pull/168505