[Attributor][NFCI] Merge MemoryEffects explicitly

We had some custom handling for existing MemoryEffects but we now move
it to the place we check other existing attributes before we manifest
new ones. If we later decide to curb duplication (of attributes on the
call site and callee), we can do that at a single location and for all
attributes.

The test changes basically add known `memory` callee information to the
call sites.