Drive-by-fixes: Singleton ranges past EOF, disable optimization
for block count mode.

Bug: v8:6000
Change-Id: I718891f8821285ce3d7d8360faaa91a43de5b93d
Reviewed-on: https://chromium-review.googlesource.com/541300Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46168}