[PATCH] Fix force_to_mode not to modify in place the passed rtl (PR rtl-optimization/58726)

[Eric Botcazou]

> I'd worry there's other latent bugs of this nature and if we'd be better
> off avoiding the temporary sharing.  We have structure sharing rules for
> a reason -- I'd hate to think of all the code that would need auditing
> to ensure it was safe with this bogus sharing.

I wouldn't throw the baby with the bath's water here, it's one of the numerous 
PRs opened by Zhendong Su and which clearly look machine-generated.  It fails 
with 4.4.x onwards and apparently nobody noticed the problem in real life.
We know that we have latent sharing issues in the combiner because of the way 
it's designed, but they are quite rare in practice.

-- 
Eric Botcazou