A counted loop whose limit is an invariant value other than the array's
length, such as `j < n` after `const n = a.length`, gets the same fast copy
as a loop to `a.length`: one test before the loop bounds the limit by
2^31 - 1 and by each indexed array's hole-free prefix, for every array the
loop provably never resizes, and the fast copy reads those slots directly.
The length proof accepts number and boolean writes to class fields, which
run no user code, and borrowed element reads in a fast copy load the slot
without the prefix test or the runtime call. The fast copy's loop body may
hold up to 160 IR nodes.
Chris Tate
3800a7d311