Incorrect bitfield aliasing with Tree SSA

[Richard Kenner]

> Uh, except as we've discovered, the RTL uses alias set 0, so whatever
> alias set you choose for these doesn't matter anyway to the RTL level.

Only in some cases.  That was a kludge put in to fix some obscure bug and
left there. I hope we can remove it at some point, and think we can.

> No i mean the idea of making it a different alias set than the parent,
> but a subset of the parent.

Because it *is* the parent in most places, so it should be in all.