RFC: merging MEM_ATTRS

[Joern Rennecke]

>     Well, we have to change that in order to get better alias information
>     of non-addressed variables.
> 
> 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, although it is true that there
has to be a way to express this: this only happens when a mem can reference
every single one of the variables that don't have their address taken, plus
something else.  We can enumerate the variables just fine with a chained
tree, but there is no way to put a NULL_TREE into a chain so that is
distinguishable from the end.
A chained tree also has the problem that we couldnt share the individual
MEM_EXPRs because of the chain links.
Both of these problems can be overcome when we use a tree_vec to
aggregate the alternatives.