It can happen that coverage infos for a function containing
IncBlockCounter bytecodes can be deleted (e.g. by switching to
best-effort coverage). Handle this case correctly in the IncBlockCounter
runtime function.

BUG=v8:6000

Change-Id: I49b9f52822661150d55410d6b173b3929adf4af2
Reviewed-on: https://chromium-review.googlesource.com/558039
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46351}