[llvm] [SafeStack] Don't move a scalable byval argument to the unsafe stack (PR #225067)

[Jakob Koschel via llvm-commits]

https://github.com/jakos-sec commented:

Is this something that can happen realistically?

I've done some tests with a minimal IR test:

```
define void @f(ptr byval(<vscale x 4 x i32>) %p) {
  ret void
}
```

And this also fails without safestack with `llc -mtriple=aarch64-linux-gnu minimal_byval.ll`.

It sounds more like this behaviour is already broken in a bunch of passes and perhaps should be rejected by `opt -passes=verify minimal_byval.ll `?

https://github.com/llvm/llvm-project/pull/225067