Auto merge of #113892 - RalfJung:uninit-undef-poison, r=wesleywiser

clarify MIR uninit vs LLVM undef/poison

In [this LLVM discussion](https://discourse.llvm.org/t/rfc-load-instruction-uninitialized-memory-semantics/67481) I learned that mapping our uninitialized memory in MIR to poison in LLVM would be quite problematic due to the lack of a byte type. I am not sure where to write down this insight but this seems like a reasonable start.

Author: bors