For the fast case we can avoid the instance type check, since the map
check covers that. We also don't need to call out to the ToBoolean
builtin in general, but just use the BranchIfToBooleanIsTrue logic.

Plus in the fast case, we don't know that the JSIteratorResult::done is
a boolean, since the map doesn't guard this assumption, so we also need
to do a proper BranchIfToBooleanIsTrue in that case.

Bug: v8:5269
Change-Id: I36f0d0841472c02f8030f9ce067d20326c9388bd
Reviewed-on: https://chromium-review.googlesource.com/778882Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49482}