RFC: merging MEM_ATTRS

[Joern Rennecke]

> 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?

I can see how it gets problematic when we have such code.  So we must record
the information that we know.  In order to make the information in MEM_EXPR
useful for alias analysis purposes, 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.