check 2:length(...) loop invariant

[ben-schwen]

I changed another 2:length(...) to this seq.int() form, but I see 2:length(...) elsewhere and now I'm second-guessing -- I guess it's fine (better, even) to use the shorter 2:length(...)?

Originally posted by @MichaelChirico in #7401 (comment)

We should check our whole codebase about this code smell and check if the invariants of length(...)>= 2 can be verified to not run into the 2:1 case