[llvm] [Local] Only intersect llvm.access.group metadata if instr moves. (PR #115868)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Before we start relying on this in one way or the other, I'd like to have it spelled out that this is UB (rather than poison) in LangRef, so we don't have to guess next time.

Tried to do so in https://github.com/llvm/llvm-project/pull/116220

https://github.com/llvm/llvm-project/pull/115868