RFC: merging MEM_ATTRS

[Richard Kenner]

    that means when we have a pointer dereference or array access, instead
    of punting and putting NULL_TREE there, we can use a special marker
    tree there (preferrably a shared one) that says that some indirection
    is involved at source-code level, and hence this can't mean any
    variable that didn't have its address taken.

That's not the case for an array acess per se since that references the
array variable.  I think it's just the case for a dereference and there's
certainly no problem with something like an INDIRECT_REF of an ERROR_MARK
or some such.  In that case, merging two different MEM_EXPRs to zero would
lose information, but I suspect the loss would be trivial in practice.