bug#64391: buffer narrowing slowdown regression in emacs 29

[Gregory Heytings]

I pushed a proposed change to the scratch/bug64391 branch.

It contains three commits:

- b741dc7fcd which is the minimal possible commit to fix this bug

- 01fb898420 which moves a few statements from the callee 
(internal--label-restriction) to its only caller 
(internal--labeled-narrow-to-region), and simplifies the code accordingly

- c52ade305e which is optional, and makes the symmetrical change for the 
widening case

Eli and Mattias, could you perhaps have a look?

Thanks.