[llvm] [InstCombine] Simplify and/or of icmp eq with op replacement (PR #70335)

[Nikita Popov via llvm-commits]

nikic wrote:

I believe the problem is this:
```llvm
define i1 @test(ptr %arg, ptr %arg2) {
  %icmp = icmp eq ptr %arg, %arg2
  %call = call i1 @llvm.is.constant.i1(i1 %icmp)
  %and = and i1 %call, %icmp
  ret i1 %and
}

declare i1 @llvm.is.constant.i1(i1)
```

The `is.constant` check gets folded away. I'd normally say this is just a QoI issue, but it seems that the libstdc++ implementation relies on this in some way I don't fully understand (there is a specialization of `std::distance` on `std::list` for getting the size of the full list, and then that is used in `_Finalize_merge` where we are currently in the process of updating list sizes, or something like that).

So I guess we should exclude llvm.is.constant from being folded by simplifyWithOpReplaced.

https://github.com/llvm/llvm-project/pull/70335