This is needed to ensure that code can be interrupted. It will be
covered by a test once we support if-constructs in Liftoff.

Drive-by: Separate handling of blocks and loops, as there is only one
line in common.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Ic22ca5e65c8d03a5d504289ec2a9e30cb97dc220
Reviewed-on: https://chromium-review.googlesource.com/853858
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50403}