Minor DOM Cleanup

[Jeffrey A Law <law at redhat dot com>]

This patch moves folding of constant array references out of DOM (where
it never belonged) and into the children of fold_stmt.  

And, yes, it's careful not to muck up stuff like &"foo"[2] and
similar constructs.  Basically it makes note of when we're
inside an ADDR_EXPR and disables constant array reference folding
in that case.

In my test bucket it picks up the same set of constant array references
as mainline does without this patch.  We actually get slightly better
code in one case -- by optimizing the constant array reference earlier,
a useless conditional is exposed earlier, which in turn exposes a 
related dead memory load early enough that the optimizers get a chance
to remove it.


Bootstrapped and regression tested on i686-pc-linux-gnu.

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