[llvm] [VectorCombine] Allow shuffling between vectors the same type but different element sizes (PR #121216)

[via llvm-commits]

github-actions[bot] wrote:

<!--LLVM CODE FORMAT COMMENT: {undef deprecator}-->


:warning: undef deprecator found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

``````````bash
git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' ac8bb7353a7fe79cd99b3c041d5a153517c31abc bcc7fb04d0199daa71050a51037e3897dbebdf80 llvm/test/Transforms/VectorCombine/X86/extract-insert-poison.ll llvm/test/Transforms/VectorCombine/X86/extract-insert-undef.ll llvm/test/Transforms/VectorCombine/X86/extract-insert.ll llvm/lib/Transforms/Vectorize/VectorCombine.cpp llvm/test/Transforms/VectorCombine/X86/load-inseltpoison.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/Transforms/VectorCombine/X86/extract-insert-undef.ll

[Undef](https://llvm.org/docs/LangRef.html#undefined-values) is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields `undef`. You should use `poison` values for placeholders instead.

In tests, avoid using `undef` and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:
```llvm
define void @fn() {
  ...
  br i1 undef, ...
}
```

Please use the following instead:
```llvm
define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}
```

Please refer to the [Undefined Behavior Manual](https://llvm.org/docs/UndefinedBehavior.html) for more information.



https://github.com/llvm/llvm-project/pull/121216