[tree-ssa] fix opt/13798

[Jan Hubicka]

> The problem shown by this case is similar to that addressed recently in
> the C++ front end wrt static initializers.  It really really sucks to
> generate 40,000 assignment statements in order to initialize a large array.
> 
> This duplicates some logic that expr.c uses in the tree->rtl converter
> to decide to drop such large constructors into readonly memory.  We then
> memcpy from the readonly memory to the variable.
> 
> One change from the expr.c logic is that, if the variable itself is
> marked readonly, then the variable is promoted to be static.  This 
> eliminates the need for a memcpy at all.  I can't find any downside
> to this promotion, so it is done unconditionally, no matter the size
> of the constructor.
> 
> This does highlight a flaw in the cgraph code in that it cannot remove
> unused local static variables.  Primarily because it doesn't even see
> these variables until after it has decided all variables must be emitted.
> I'm not sure how to address this, exactly.

I think the proper sollution would be to simply deal with local static
variables as if they were global by frontends, but I didn't get across
to implement this yet.

Honza