Improve regex compiler's arc moving/copying logic.

The functions moveins(), copyins(), moveouts(), copyouts() are
required to preserve the invariant that there are no duplicate arcs in
the regex's NFA.  Spencer's original implementation of them was O(N^2)
since it checked separately for a match to each source arc.  In commit
579840ca0 I improved that by adding sort/merge logic to be used if
more than a few arcs are to be moved/copied.  However, I now realize
that that missed a bet.  At many call sites, the target state is newly
made and cannot have any existing in-arcs (respectively out-arcs)
that could be duplicates.  So spending any cycles at all on checking
for duplicates is wasted effort; in these cases we can just blindly
move/copy all the source arcs.  Add code paths to do that.

It turns out that for copyins()/copyouts(), *all* the call sites have
this property, making all the "improved" logic in them flat out
unreachable.  Perhaps we'll need the full capability again someday,
so I just #ifdef'd those paths out rather than removing them entirely.

In passing, add a few test cases to improve code coverage in this
area as well as in regc_locale.c/regc_pg_locale.c.

Discussion: https://postgr.es/m/810272.1629064063@sss.pgh.pa.us

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