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

[Nikita Popov via llvm-commits]

nikic wrote:

I've been thinking about this again, and I think a clean way to handle this may be via operand bundle.

```llvm
; Non-volatile memset
call void @llvm.memset(ptr %p, i8 0, i64 %len)
; Volatile memset
call void @llvm.memset(ptr %p, i8 0, i64 %len) ["volatile"()]
```

Operand bundles are allowed to add additional memory effects, so this would be consistent with our overall IR model.

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