fix: use V(graph) as we should

[maelle]

Fix #1432
Fix #1427

cc @krlmlr @clpippel

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[krlmlr]

I still think this should be

Suggested change

	identical(index, V(graph)))
	identical(index, as.numeric(seq_len(vcount(graph)))))

We want to cater for cases where !missing(index) but index is still essentially V(g) . I see:

library(igraph)
g <- make_ring(10)

identical(V(g), V(g))
#> [1] FALSE

^{Created on 2024-07-18 with reprex v2.1.0}

Similarly to #1432 (comment).

[clpippel]

graph <- make_ring(5)
index <- V(graph)
identical(index, as.numeric(seq_len(vcount(graph))))
# returns FALSE.

Try all(index == V(graph))

[krlmlr]

@clpippel: all() is challenging if the vectors are of different length? We'd still want to check the lengths first.

Would the all() route be faster?

[krlmlr]

Perhaps we also want to check isTRUE(attr(g, "is_all")) ?

options(conflicts.policy = list(warn = FALSE))

library(igraph)
g <- make_ring(10)
dput(V(g))
#> structure(1:10, class = "igraph.vs", is_all = TRUE, env = <weak reference>, graph = "516d6a01-f041-48ae-bbe3-60dd751a8f4b")

^{Created on 2024-08-22 with reprex v2.1.0}

Need to check if this other code path is ever touched -- easiest way by breaking it and running tests or revdeps.