[llvm] [LangRef] Specify semantics for non-byte-sized loads and stores (PR #180739)

[Calvin Beck via llvm-commits]

Chobbes wrote:

This might be addressed in the new byte proposal, but I do wonder how this interacts with loads from uninitialized memory. My understanding has been that if I load uninitialized memory I currently get `undef`, but would I get poison if I loaded an `i20` instead? How would this interact with the new byte types?

https://github.com/llvm/llvm-project/pull/180739