[PATCH, PR70045] Unshare create_empty_if_region_on_edge argument

[Tom de Vries]

Hi,

this patch fixes PR70045, a graphite 6 regression.

The problem is as follows: in graphite_create_new_loop_guard, a 
condition cond_expr is constructed using an upper bound expression *ub.

During the call:
...
     exit_edge = create_empty_if_region_on_edge (entry_edge, cond_expr);
...
the cond_expr is modified in place, which has as side-effect that *ub is 
modified.

The patch fixes this by unsharing the cond_expr before passing it as 
argument to create_empty_if_region_on_edge.

Bootstrapped and reg-tested on x86_64.

OK for stage4 trunk?

Thanks,
- Tom
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