`for (let i = start; i < end; i++)` kept a double counter when the start had
no proven integer range, as in a hash over `[start, end)` whose bounds come
from fields. Such a loop is now versioned like one with an unproven limit:
when the start is an exact integer of at most 2^53 - 1 in magnitude (compared
bit for bit, so -0 takes the original loop), the integer-counter copy runs.
The usual bounds on duplicated bodies apply.
Malte Ubl
2b056343ee