[llvm] [FunctionAttrs][IR] Fix memory attr inference for volatile mem intrinsics (PR #122926)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Why aren't the function declarations correct in the first place?  If llvm.memcpy can be volatile, the intrinsic declaration should indicate that it can modify inaccessible memory.  Marking the declaration argmemonly, then overriding that later, seems backwards.

https://github.com/llvm/llvm-project/pull/122926