[FunctionAttrs] Volatile operations can access inaccessible memory

Per LangRef, volatile operations are allowed to access the location
of their pointer argument, plus inaccessible memory:

> Any volatile operation can have side effects, and any volatile
> operation can read and/or modify state which is not accessible
> via a regular load or store in this module.
> [...]
> The allowed side-effects for volatile accesses are limited. If
> a non-volatile store to a given address would be legal, a volatile
> operation may modify the memory at that address. A volatile
> operation may not modify any other memory accessible by the
> module being compiled. A volatile operation may not call any
> code in the current module.

FunctionAttrs currently does not model this and ends up marking
functions with volatile accesses on arguments as argmemonly,
even though they should be inaccessiblemem_or_argmemonly.

Differential Revision: https://reviews.llvm.org/D135863