RFC: merging MEM_ATTRS

[Richard Kenner]

    > No.  As I said, this can be done with alias sets or with some flag
    > or some such.  We must have a way in a MEM_ATTRS of saying "we don't
    > know what this references".

    It's actually quite rare that we need this, 

No.  You need this whever you haven't bothered to set MEM_ATTRS.  There's
absolutely no requirement that all MEM's have it set and there are lots
of cases where they don't.  Are you proposing to treat each of them as bugs?