Instead of changing the [[IteratedObject]] field to undefined to mark an
array iterator as exhausted, store the appropriate maximum value into
the [[ArrayIteratorNextIndex]] field such that the iterator will never
produce any values again.

Without this change the map check and the "length" access on the
[[IteratedObject]] cannot be eliminated inside the loop, since the
object can either be the array or undefined. Even with this change
it's still not possible immediately due to missing aliasing
information in the LoadElimination, but it paves the way for follow
up improvements. Eventually the goal is to have `for..of` as fast as
a traditional `for` loop even for really tight loops.

This CL also hardens the implementation of the ArrayIterator by using
proper CASTs and CSA_ASSERTs. The readability of the CSA builtin was
improved by utilizing proper helper functions.

Bug: v8:7510, v8:7514, v8:8070
Change-Id: Ib46604fadad1a0f80e77fe71a1f47b0ca31ab841
Reviewed-on: https://chromium-review.googlesource.com/1181902
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55261}