[regex] Remove empty_set

[Paolo Bonzini]

Several places use a dummy node_set instead of avoiding loops entirely.
This has a mild (1%) effect on performance and makes the code less clear
IMO, hence this patch.  It also prepares for the following patch.

I took the occasion to clean up sift_states_backward, moving the inner loop
out of the function into a new build_sifted_states.

Paolo

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