[llvm] [WebAssembly] Change placeholder from `undef` to `poison` (PR #131536)

[Pedro Lobo via llvm-commits]

pedroclobo wrote:

> Are there any differences in these two options in this case? I think the value is used only as a placeholder that is to be replaced later.

Conceptually, as the value is later replaced, there is no difference between using `poison` or `undef` here. 
Nonetheless, we should prefer `poison`, helping the [ongoing effort to remove `undef` and replace it with `poison`](https://llvm.org/docs/UndefinedBehavior.html#undef-values).

https://github.com/llvm/llvm-project/pull/131536