nir/opt_loop_unroll: Fix has_nested_loop handling

In 87839680, a very subtle mistake was made with the CFG walking recursion. Instead of setting the local has_nested_loop variable when process child loops, has_nested_loop_out was passed directly into the process_loop_in_block call. This broke nested loop detection heuristics and caused loop unrolling to run massively out of control. In particular, it makes the following CTS test compile virtually forever:

Fixes: 87839680 "nir: Fix breakage of foreach_list_typed_safe..."

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