[llvm] [InstCombine] Convert load from LUT into a select (PR #98339)

[via llvm-commits]

erikdesjardins wrote:

> Miscompilation with undef: [alive2.llvm.org/ce/z/JL6br](https://alive2.llvm.org/ce/z/JL6br)_
> 
> ```
> @.str = constant [2 x i8] [i8 1, i8 undef], align 1
> 
> define i32 @src(i64 %x) {
> entry:
>   %arrayidx = getelementptr [2 x i8], ptr @.str, i64 0, i64 %x
>   %0 = load i8, ptr %arrayidx, align 1
>   %conv = zext i8 %0 to i32
>   ret i32 %conv
> }
> 
> define i32 @tgt(i64 %x) {
> entry:
>   %cond = icmp eq i64 %x, 0
>   %conv = select i1 %cond, i32 1, i32 undef
>   ret i32 %conv
> }
> ```

Hmm, I would expect this transform to generate

```
define i32 @tgt(i64 %x) {
entry:
  %cond = icmp eq i64 %x, 0
  %conv = select i1 %cond, i8 1, i8 undef
  %zext = zext i8 %conv to i32
  ret i32 %zext
}
```

which is valid (https://alive2.llvm.org/ce/z/ewQnvE).

Is there some existing bad transform (folding zext into select arms) which caused that miscompile?

https://github.com/llvm/llvm-project/pull/98339