[test] TODO the 3 remaining failing -O2 tests

Each one with a ticket. Still trying to fix them though.
Looks easy enough.
Add -O2 to t/fullharness. Save to run now, to detect further
regressions.
[imcc] fix -O2 check types in constant propagation #1043

set reg, const is only a valid const setter ifregx and const have the same type,
otherwise the type are coerced, like set N0, 1 or set I1, 1.5.
Do not propagate such values.
Fixes t/op/jitn_13.pir [GH #1043]
[imcc] fix -O2 empty ins->next in used_once [GH #1042]

If the deleted op leads to the last op, which has no op->next
break the used_once loop.

Fixes GH #1042 t/op/exit_6.pasm
    set I0, 0 # used_once => deleted
    end