RFC: merging MEM_ATTRS

[Richard Kenner]

    > I think that it's simpler just to use zero: the information isn't
    > that useful by then.

    It isn't that useful to enable more optimizations, but necessary to avoid
    incorrect ones, as discussede here:

    http://gcc.gnu.org/ml/gcc-patches/2004-01/msg03234.html

I don't see the discussion from there, but certainly something is
wrong: zeroing a field like MEM_EXPR means you don't know anything, so
I don't see how doing do can produce an incorrect optimization.

    If we zero the MEM_EXPR if one or more of the alternatives accesses a
    variable that never has its address taken and that we reasoned thus
    can only be accessed with explicit accesses, we would incorrectly
    assume the MEM can't affect/use the variable any more.

I don't follow.