[FunctionAttrs] Only check ArgMem effects when inferring argument attrs (#69571)

When inferring readonly/writeonly on arguments, if the argument is
passed to a call, we should only check the ArgMem effects implied by the
call -- we don't care whether the call reads/writes non-arg memory
(captured pointers are not relevant here, because they will abort the
analysis entirely).

This also fixes a regression that was introduced when moving to
MemoryEffects: The code was still checking the old WriteOnly attribute
on functions, which no longer exists.