objects_must_conflict_p

[Richard Kenner]

    I think that's overly optimistic: what will ensure that an operation on a 
    field of the first aggregate will always conflict with an operation on a 
    field of the second aggregate, if they happen to be given the same address 
    on the stack? Nothing I'd say, like in PR optimization/11198.

I think you're right.  It would be enough to check if the alias sets have
a common subset.

    So, unless we want to recursively check that every member of the first
    aggregate conflicts with every member of the second aggregate that
    will overlap it on the stack, we should return 0 too for aggregate vs
    aggregate.

In which case, we essentially *never* share stack slots since they aren't
allocated for scalars.