[PATCH] Make alias_sets_conflict_p less conservative

[Richard Kenner]

> This makes alias_sets_conflict_p less conservative.  rev 34373 (Kenner)
> added a check that makes a structure containing a member with alias
> set zero alias everything.  Which IMHO is non-obvious and pessimizes
> for example the testcase in PR27799.  Richard, can you explain this
> change?

I'm not certain, but I think it goes like this: if a structure has
a member with alias set X (nonzero), we normally mark the alias sets
in such a way that the alias set of the structure as a whole conflicts
with alias set X.  But if X is zero, the subsetting mechanism doesn't
allow doing that.  So we have to do it explicitly, like in this code.

In other words, this is exactly analogous to the non-zero alias set
of a field case.