[Bug debug/90231] ivopts causes <optimized away> iterator in the loop

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90231

--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Note, there already is a function that is meant to handle this,
remove_unused_ivs, just either it doesn't find the right IV candidate, or
get_computation_at fails.
On this particular testcase, I see 2 IVs in the code originally, with the PHI
results base_17 and i_18, and two IVs in the code afterwards, base_17 and
ivtmp.11_20.  remove_unused_ivs for i_18 decides the best candidate is base_17,
which is unsigned int IV with step 15, so I'd guess the problem we've discussed
happens for that one.  That base_17 cand has cand_pref of 2, while the
ivtmp.11_20 one has cand_pref of 0 and so base_17 wins.  Anyway, just trying to
pass the other cand to get_computation_at fails too.  Why?